Jar opened once - is it still good for pickling? I have recently been making pickled beetroots and they mostly came out fine except for one jar which has failed to get a vacuum seal. Now I put it back in the pan and simmered the jar for a good half hour, but that didn't seem to change anything. 

I then gave up on it and thought that I might as well just eat the beets in the upcoming week. And so I opened the jar and took out one slice and then put the jar in the fridge. To my surprise, after a few hours the jar has actually sealed itself, but I have no idea what to do with it.

Is it still safe to pickle it as normal after it has been opened once? Or should I keep it in the fridge and eat it soon?
Keep it in the fridge and eat it soon.  There is a chance that you contaminated the product when you  opened the jar and took out a slice.  It is no longer sterile.