The Bastille, the great prison/fortress in the heart of Paris which was stormed by a Parisian mob on the 14th of July 1789, was attacked because they wanted its gunpowder and weapons.
The French Revolution itself began for a multitude of reasons. First, there was a major economic crisis, caused by the successful French intervention in the American War of Independence. There had also been a string of bad harvests, leading to near famine, and hungry people agitate better. Many French soldiers who had served in the American Colonies had also been in close contact with American Rebels, and so brought their ideas back to France with them when the war ended. There was also wider influence of Enlightenment thinking, such as the idea of the Social Contract where the King was allowed to rule (with varying checks and balances on his power depending on which specific theory we are talking about) in exchange for keeping his people and their property safe. The idea was pretty dependent on the King getting his power from the people, rather than from God. And if the people gave him his power, then the people could take it away.
Perhaps xenophobia also played a part, as the King’s wife was Austrian. Austria had historically been a major rival to France, but had allied with them in an ally swap with Prussia and Britain in the 1750s. Marie Antoinette was initially popular, but that popularity did not last and she became an easy target for anti-monarchical sentiment. There was also the excessive taxation heaped upon the lower classes.
There were only seven prisoners in the Bastille, and at least one of them was locked up again afterwards.
