Astromici has good advice. You can also try to restore your old save file if that doesn't work and you suspect you have a corrupt save. To restore a save file, please follow these steps:
Navigate to-
\My Documents\Electronic Arts\The Sims 3\Saves
and you should see save files of your towns/Sims along with copies that say .backup.
Example-
riverview.sims3 and riverview.sims3.backup
Delete the original save file (riverview.sims3 in this example) and erase ".backup" from the other filename, and you should be able to revert to your last full save.
Hope that helps!