increased demand but no increase in price? I am teaching myself basic economics and I'm trying to understand supply and demand. 
Recently, in my country, there was news of a strike by the petroleum suppliers. Expectations of an impending fuel shortage increased demand. (as evidenced by the long queues at petrol stations).

Now,by plotting the supply and demand curves, it seems apparent that an increase in demand would lead to an increase in equilibrium price and equilibrium quantity. However, in reality, during this time, none of the petrol stations increased the price of petrol. Is this because of government regulation of prices or due to some other factor? 
Thanks
Such a situation can be explained in the following ways:


If supply is perfectly elastic (horizontal supply curve), then an increase in demand does not increase the price. It's highly unlikely that supply of petrol here is perfectly elastic. This could be the case under perfect competition with constant marginal costs of petrol. However, typically petrol has increasing marginal costs. Alternatively, it could be the case with constant marginal costs and Bertrand competition (price competition, with few firms) meaning they also set prices equal to marginal costs as in perfect competition.
There may be costs to increasing the price. So-called menu costs make prices sticky and therefore changes are not as often as we expect. E.g. if you're a restaurant and want to increase prices, you have to reprint all menus, which may not be worth it for a temporary demand increase. However, this is probably not tbe case in your example.
When setting the price before the shortage was announced, the increase in demand was expected and the price already set accordingly high. Was there an increase in the price (shortly) before the announcement? If not then the next explanation is the only possibility.
Government regulation of the price.