How to Fix a Slipped Bike Chain
To fix a slipped bike chain, first, turn your bike upside down, as this will make it a lot easier to reattach the chain. Once your bike is upside-down, look at the derailleurs, which are the small machines on the front and back gears that move the chain. Note which gear these are on so you can place the chain back in the right place. Then, push the rear derailleur arm toward the handlebars, which will give your chain some slack. With your other hand, slide the chain back onto the right gear and slowly release the derailleur after you have 10 to 15 teeth in place. To get the rest of the chain on, move the bike pedals backward slowly for 1 full rotation.
Inspect the drive chain first. If there are any bent or broken pieces, repair or replace these before you reattached the slipped chain. Make sure to examine not just the chain itself but the derailleurs and cassette (which are the gears). This is especially important to do after a crash to make sure that there is no damage. After fixing the bike chain, it is a good idea to inspect the cassette, derailleurs, and limit screws to see if they are properly adjusted or to check if they need replacement. Turn the bike upside down or hang it from a bike stand. This will make fixing the chain easier and prevent the bike from rolling around while you work. Flip the bike so that it rests on the seat and handlebars, setting it down gently to prevent scratching or scuffing. Bike stands hold the bike right-side-up in the air and are great for maintenance. However, since most chains slip on the road, you will likely not have access to one. Instead, you may set the front of the seat over a large horizontal pole. Keep the bike about 4 feet (1.2 m) off the ground or at least high enough that the rear wheel is not touching the ground. Note what gear the bike is in. Derailleurs are the small machines on your front and back gears that physically move the chain from gear to gear. Note where it is currently resting by looking down the derailleur -- it will be lined up with the gear. You want to put the chain back on this gear. The front derailleur, right next to the pedals, looks like a little metal bracket that hovers over the gear the chain should be on. The rear derailleur, found by the back wheel, looks like a small mechanical arm. This arm slides back and forth underneath the cassette (collection of gears) to move the chain. It will be underneath the right gear. Many bikes will tell you the gear number on the handlebars, but you need to know how to read them to make sense of it: The left hand adjusts your front gears : 1 is the gear closest to the bike, or smallest gear. The right hand adjusts the back gears : 1 is the gear closest to your bike, which is the biggest gear. Push the rear derailleur arm toward the handlebars to get slack on the chain. This is the small metal arm next to the cog on the bottom of the derailleur. There is usually a little metal square right next to the cog that lets you push the derailleur without getting too greasy. It should fold gently towards the front of the bike so that the chain hangs with a lot of slack. Use your other hand to slide the chain back on the right gear. Pick up the chain with 2-3 fingers and drape it on top appropriate rear rear gear. Bring the other end of the chain to the bottom of the front gear and hold it there. You should have enough slack to get 10-15 teeth on the gear into the groves of the chain. Slowly release the derailleur once you have some teeth in place. You can also use a pencil, pen, or small stick to guide the chain onto the gears if you don't want to use your fingers. Pedal the bike backward slowly for 1 full rotation with your hand. As you pedal, you'll notice that the teeth you attached manually will guide the rest of the chain back into place. After the first rotation, pedal forward for 2-3 more rotations to make sure that the chain is securely in place. Make sure you are pedaling the correct direction -- the back wheel will move as you pedal forward, but not backward. Take care of your drivetrain to prevent chain slippage. The drivetrain is the transmission of your bike. It consists of all the parts that get your back wheel moving: chain rings (big gears next to your pedals), a cassette (collection of gears on your back wheel), a rear derailleur (metal arm on the back wheel), and the chain itself. As dirt, grit, and grime collect on your drivetrain, it wears down and becomes prone to skips and slips. Frequent cleaning and maintenance of your drivetrain can add years to your bike's working lifespan. You will need to turn the bike upside down or clamp it in a bike rack, in order to work on the drive train. Use an old rag and some bio degreaser to scrub the chain. Biodegreaser, sometimes called biodegradable solvent, is a powerful soap that cuts through grime but won't ruin your chain. Most bike stores sell it next to the chain lube. If you don't have any, you can use isopropyl alcohol. Pour a little bit on a damp rag and clamp it lightly over the chain with one hand. Use the other hand to pedal the bike, running the chain through your rag for 2-3 cycles. Go through 2-3 cycles putting pressure on the top and bottom of the chain, then another few putting pressure on the sides. Lightly scrub away any patches of grease or grime with your rag if you still see them. Use a bicycle brush or old toothbrush to clean your gears. Just like human teeth, gears need to be flossed from time to time. Dip your brush into the biodegradable solvent or isopropyl alcohol and run it in between each gear as you pedal with the other hand. This removes clumps of grease that can dislodge your chain if they are allowed to get too large. Use a screwdriver to scrape off hard to reach areas or precise, small spots. A screwdriver is ideal for getting rid of any grime on the pulleys on the rear derailleur Wipe away any visible grime on the derailleur and chainrings. If it looks dirty then you should get rid of it. Use your damp rag, brush, and a little degreaser to get into as many nooks and crannies as you can and get your bike sparkling clean. Let the bike do the work for you whenever possible, holding the rag/brush in place while spinning the pedals. Common areas to focus on include: Both sides of the idler pulley and jockey pulley wheels, which are the small cogs on the derailleur arm. The back side (closest to the bike) of the chainrings. The bike frame, joints, and hinges near the chain. Purchase a chain cleaner for seriously grimy chains. If a rag and a toothbrush don't cut it, you'll need a chain cleaning tool. These little boxes clamp over your chain. You add degreaser and hold the tool in place while pedaling the bike backward, allowing it to automatically brush and scrub the chain links for you. They are only $20-$30 and frequently come with degreaser and a brush. Lube your bike chain after cleaning it. Purchase a bottle of chain lube, which should both lubricate the chain and protect it from dirt and moisture. After cleaning and drying the chain with your rag, slowly turn the pedals. Apply a single drop of lube to every 2-4 links, right on the joint where one link meets another. After you've gone through the whole chain, shift through your gears and apply another 10-12 drops. Use a clean rag to wipe up any excess lube from the chain when you are done, as extra lube can hold dirt and lead to grime. Your goal is to get a light coating of lube on the entire chain. Anytime you ride in the rain, clean the chain, or hear squeaking, you should apply lube. Feel the chain with your fingers -- if it feels dry then you need to apply more lube. Learn to shift properly to keep your chain in place on climbs. Improper shifting puts a strain on your drivetrain, and your chain can only stretch so far before it slips or potentially breaks. Shifting moves the chain, and if you are cranking on the pedals up a hill this can cause it to miss the teeth of the next gear. Some tips for safe shifting include: Shift before you get to a hill. Don't wait until you can barely pedal to shift. As a general rule of thumb, your feet should always move at the same speed -- you keep shifting to make this possible. Use "soft pressure" when you shift. Right as you shift, ease up on your feet as if letting go of the gas. You don't want to stop pedaling, you just want less weight on the pedals. Work on timing this with your shift, then resuming normal pedaling. Adjust your limit screws if the chain always falls out in the same direction. This is common if, when shifting to one of your "extreme gears" on the far side of either set of gears, the chain keeps going and slips off. Limit screws tell the derailleur to stop moving in one direction, and if the limit is too wide, the chain will keep moving when you shift even though there is no gear to catch it. Both the front and back derailleur have tiny limit screws labeled "H" and "L" for "High" and "Low" limits. Turn the "H" screws clockwise to prevent the chain from moving too far to the right, away from the bike. Turn the "L" screws clockwise to prevent the chain from moving too far to the left and into your wheel in the back or to keep it from falling between the seat tube and chain ring in the front. If you are in the furthest gear, you will see the derailleur moving as you adjust the screws. Make sure it lines up in the middle of the gear. Replace worn out chains or those with broken or bent links. With age, both chains and cassettes will wear down due to friction, which means the gear teeth cannot lock firmly into place in the chain. To check the chain, use a ruler to measure a 12 in (30 cm) distance between 24 pins in the chain. The pins are the small circles in the middle of the links when you look at the chain from the side. If the twelfth pin is more than ⁄ 16 inch (0.16 cm) off the 1 foot (0.30 m) mark, you need a new chain. It's better to replace a chain with bent or sticking links than try to replace individual links. Replacing a single link in a chain means the links won't have the same amount of wear, which could be dangerous. If you absolutely must replace a link, make sure it is the same brand as your chain and meant for a bike with the same number of speeds. If your chain is covered in rust or the links have trouble moving, it is best to purchase a new chain. Chains generally wear out faster than cassettes and are much cheaper to replace. Determine if you need a new cassette. Cassettes are harder to diagnose than chains, but if you feel like you need a new one, you likely do. If your chain is constantly skipping over gears, sliding off, or slipping as you pedal, you likely need a new cassette. Take your bike to the shop if there is any confusion for a personal appraisal. After cleaning the cassette, look at the gears. Do any of them look more visibly worn down than others? If there is a disparity it is likely time to get a new cassette. 
Inspect the drive chain first. Turn the bike upside down or hang it from a bike stand. Note what gear the bike is in. Push the rear derailleur arm toward the handlebars to get slack on the chain. Use your other hand to slide the chain back on the right gear. Pedal the bike backward slowly for 1 full rotation with your hand. Take care of your drivetrain to prevent chain slippage. Use an old rag and some bio degreaser to scrub the chain. Use a bicycle brush or old toothbrush to clean your gears. Wipe away any visible grime on the derailleur and chainrings. Purchase a chain cleaner for seriously grimy chains. Lube your bike chain after cleaning it. Learn to shift properly to keep your chain in place on climbs. Adjust your limit screws if the chain always falls out in the same direction. Replace worn out chains or those with broken or bent links. Determine if you need a new cassette. 