Sounds like your library database may be being corrupted. With iTunes 11 an "empty" library will show the previous purchases of the account that is currently signed in. To prevent this go to iTunes Preferences > Store and untick Show iTunes in the Cloud purchases. Have you actually checked the hard drive to see if the media is still present? Chances are you need to restore an older library file in the way set out below...
Empty/corrupt library after upgrade/crash
Hopefully it's not been too long since you last upgraded iTunes, in fact if you get an empty/incomplete library immediately after upgrading then with the following steps you shouldn't lose a thing or need to do any further housekeeping. In the Previous iTunes Libraries folder should be a number of dated iTunes Library files. Take the most recent of these and copy it into the iTunes folder. Rename iTunes Library.itl as iTunes Library (Corrupt).itl and then rename the restored file as iTunes Library.itl. Start iTunes. Should all be good, bar any recent additions to or deletions from your library.
The images are from Windows XP, but the principle is the same. Depending on the age of you library the library file may not include the .itl extension.
tt2
