I'm not sure there is a guide somewhere in this forum already, but basically the trick is this:
In your Documents, you'll find a The Sims 3 folder. This is the game folder, which holds all your saves, CC, settings etc. You'll find your save games in the Saves subfolder there.
If you delete that folder and start the game, the game will create a brand new one, and basically reset to factory defaults. However, this will also get rid of all your settings, saves and CC.
In this case you do not want that, so what you do is RENAME the folder instead of deleting it. If you start the game, it will not find the folder and create a new one, just as above, only you'll still have your stuff in the renamed folder.
The interesting thing here is to see whether the game will actually start now. Whether it does or does not determines the next action to take.