In your Saves folder (Documents > Electronic Arts > The Sims 3 > Saves) you should find saves labeled "whatever.sims3" and "whatever.sims3.backup". You may see "whatever.sims3.bad" - if you see those, you should delete them.
Basically, locate your Save Folder, delete it, and rename it's backup folder from "whatever.sims3.backup" to "whatever.sims3". Remember when you first saved the game, and it prompted you for a name? That's what it will be called. If you named it "crimsonfyre", the folders you are after are "crimsonfyre.sims3" and "crimsonfyre.sims3.backup".