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.
Alternatively, depending on exactly when and why the library went missing, there may be a more recent .tmp file in the main iTunes folder that can be copied and renamed as iTunes Library.itl to restore the library to an earlier state. Look for a recent .tmp file that is similar in size to the .itl files in the Previous iTunes Libraries folder. If it has happened repeatedly you may want the earliest such file generated since the last iTunes upgrade.
If applicable, 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.
I've noticed more of these missing library posts of late and a common factor to most since I started asking is AVG Anti-Virus. It seems in some cases it might be at least part of the reason why the library file disappears. Try excluding the iTunes folder from any AV scanning process.
This is interesting. I just posted earlier today about my files disappearing after I updated AVG on my computer. Luckily, before I updated, I backed up all of my files, including my iTunes Library.itl file. I was able to copy it over the new .itl file that was put in place and all of my songs, playlists, etc. were restored.
Here is the link to the other discussion...that has been going on for over four years apparently:
It appears that AVG was the cause of my problem. At first I just made sure that any folder containing data that was in iTunes was un-checked and not set to be scanned. This seemed to work. After coming back from vacation and booting up my pc, I initially saw that my iTunes library was fine. AVG immediately kicked off a scan since I had been gone for almost a week. After the scan was complete, AVG prompted a re-boot. After re-booting, iTunes was blank again. I un-installed AVG and went with Avast. I'll report if I have any issues going forward. Thanks to all!