@Xylbit If you currently have Sims 3 installed through the EA App, please repair it (game library > Manage > Repair). This will undo any changes you've made to the program files, for example adding the graphics card to to the game's database, but won't delete any user data.
If you have a disc install, running the Super Patch will accomplish the same thing. This is a direct download link to EA servers:
http://akamai.cdn.ea.com/eadownloads/u/f/sims/sims3/patches/TS3_1.67.2.0240xx_update.exe
Some browsers flag the download because it uses http rather than https, but it's perfectly safe.
Saves and other user data are stored in the Sims 3 user folder, in Documents > Electronic Arts, so it's the important folder to preserve if you don't want to lose anything you can't get back. The folder can be moved or renamed to prevent the game from reading it, and restored later to regain access to its contents.
If this doesn't help, please post a screenshot of the error you're seeing when you try to open the game.
