MillieT03 Please remove or rename the Sims 3 folder in Documents > Electronic Arts. Maybe some file in this folder is corrupt in a way that would cause your issue. I haven't seen it happen before, but since it's technically possible, and this is an easy test, please try it. This will remove, but not delete, your saves and other user content, so don't panic when you don't see those files available.
If that doesn't help, please do a clean uninstall and reinstall of Sims 3 and the EA App, as described here (the reply):
[Read First] Running Sims 3 in Windows | EA Forums - 8323446
Create a new folder into which you'll install Sims 3, as mentioned. If you use a third-party antivirus, disable it while reinstalling the game, and don't reenable it until you've tested once. As long as you don't do anything else at the same time, your computer should be safe.