What if the French Revolution failed?
Define fail and, for that matter, define French Revolution. After all, depending on how you slice it, by 1799, 1804, 1814 or 1815, the Revolution had pretty badly failed at the whole regime-overturning deal and yet if you look at the world in 1948, or Europe in 1990, you can definitely see there are matters more important where its death could not prevent it from having its ideals propagated.
Is it that the nobles and the clergy manage to effectively muzzle the Tiers État in the États Généraux and convince the king to do the barest of reforms while working with them? That's still going to leave a massive amount of financial problems behind and the anger simmering.
Is it that the députés from the Tiers État don't take their oath to give the country a Constitution or that the Parisians fail to either rise up or to take the Bastille? In a way, the Rubicon has already been crossed and movements all around France's towns and countrysides are happening.
Is it that the Great Fear doesn't happen and the abolition of the privileges and maybe the Declaration of Rights of Man and of the Citizen don't get proclaimed in August 1789? If the Revolution radicalises at some point or other, something like them is going to occur.
Is it that the Constituante can't agree on a text? the First Coalition faces no significant opposition and waltzes into Paris? Those are unlikely and bring their own sets of problems.
Etc, etc.
The Revolution is an immensely volatile period, with lots of actors and extremely butterfly-prone because of such massive interactions but because of that, you need to be specific.