this has happened to me beofre and I never found out if this way was 100% effective in fixing it but the one time I tried it, it worked.
Ok so when you turn on your comuter if the sims 3 application automatically starts up all on its own with you opening it(like mine) then exit of of the application force quit it and make sure the application is totally exited from. Once you have done this you could go to your finder or whatever (for a mac its a finder I don't know what it is for Windows) but hopefully you know what I mean. But basically just click on the sism 3 icon whether its on your desktop of the computer or in the finder thing either way just click on it to get it running. That worked fr me. I think Sims needs to be started by someone instead of just by itself or by the computer automatically starting it.
Hope it helps.
