Couple of possible solutions:
1. Try lauching base game from Steam. It will not go past updating database 99%, because you probably have to many mods for base game to run. But it could be enough, to correct some basic files that got corrupted. When it gets to 99% and gets stuck, you have to force close default AC game
2. Try deleting the apps folder inside AC install. You will lose all the apps on the screen inside the game, but one of the apps could of gotten corrupted. You can go the long way, first disable all apps inside CM and start race without any of them. If the race starts, exit race and start enabling apps one by one and restaring a race after you enabled the first one, second one,.... exit race after each app is enabled and the race starts, and go to the next one. When the race won't start, you will know that last app you enabled is to blame.
3. Do a reset of the config folder as described above in one post
3. If the game still don't start even after doing all those things, you may have to run integrity check in Steam (right click, find it the menu). This will reset all the default AC files. You will have to change some settings back to your liking, but it's not so much work, as people may think. All the CM stuff stays basically the same. Still hope, you don't need to get to this step.