Answer: French revolution started when General Estates were convened (1789). Such summoning of General Estates. That took place after almost 200 years because of financial crisis France of Louis XVI was suffering. The king needed to pay off kingdom´s depth. There was no money so had to raise the tax. Who was supposed to suffer was a so-called Third State (majority of population). At that time tension within society was already considerable.
Explanation: King together with aristocracy and clergy believed that - because formally they represented majority in General Estates - there would be no problem to increase taxes. But representatives of the Third State decided to establish themselves separately as "Assemblée Nationale" (National Assembly) and started to define new rules of politics. The court, aristocracy and clergy lost control. First constitutional monarchy was established (against Louis XVI´s will, of course), then republic was proclaimed and in January 1793 the king was executed (in 1794 the queen, Marie Antoinette, was executed as well). That was a revolution englightened philosophers did not plan. Many intelligent people of that were shocked what human nature is capable of.