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 renamed as iTunes Library.itl to restore the library to a recent state.
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 since I started asking is AVG Anti-Virus. It seems it might be at least part of the reason why the library file disappears. Try excluding the iTunes folder from any real-time scanning process.