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. Note that in iTunes 11 an "empty" library may show your past purchases with links to stream or download them.
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.
See iTunes Folder Watch for a tool to catch up with any changes since the backup file was created.
When you get it all working make a backup!
Should you be in the unfortunate position where you are no longer able to access your original library, or a backup of it, then see Recover your iTunes library from your iPod or iOS device.
Would you mind telling me which anti-virus package you use? It may be that it is disturbing iTunes when it tries to write out the fresh database, as it does after almost any activity with your library. I'm noticing more of this type of problem than usual and trying to see if there is any pattern. It may help to prevent a repeat of the problem if you look at the advanced options for your AV package and see if you can find a way to exclude the iTunes folder from any real-time scanning process.
tt2
