Hi Sonshine,
I've had this happen a couple times within the last six months. Restarting the computer helped.
I think it may have been my fault, so to speak. Due to me playing a different game (Minecraft) for more than a few hours, then trying to launch Sims 3 afterwords. *Blush* Computer can't handle that.
If that doesn't seem to help. Try deleting the game cache files.
MrsFlynn