What do you do if you lost a sims 3 expansion and the original sims 3 doesn't work?

First for the expansion. Have you had a really good look? I think you have to buy that disk again. If its a disc without a cover roaming around, its probably scratched and it wont work anyway.

Whats wrong with the disc? I remember my game wouldn't work because it kept freezing. If its a freezing problem reinstall the game. if its the disc, lots of video rental stores repair discs.