My daughter's Pet expansion pack keeps disappearing

My daughter's Pet expansion pack keeps disappearing. She'll access it on the cd and it will be ok for awhile, but then just is gone. I have searched her whole computer not anywhere. We want to install it again, but it says the code is already used. This doesn't make sense because if we paid for a code 1x should be able to reuse atleast on same computer for reinstalling it. Any ideas on what we can do?

her version of sims3 1.69.43.024017