I had the same thing happen to me. Try this: #1 Be sure you have iTunes updated to its' latest version. I think that would be v. 1.1 (for os 9). #2 Up the memory allocation to three times the suggested amount. (if you don't know how to do this, get back here and someone will help you.) #3 Backup your iTunes files. I had mine backed up to a separate hard drive, and I was able to retrieve the lost ones easily.
By the way, I have not had this problem for a long time, probably due to steps #1 and #2 above. Good luck!