Either you've downgraded iTunes without rolling back to an earlier version of your library file, or the library file is corrupt. Restore using the following method.
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 as iTunes Library (Corrupt) and then rename the restored file as iTunes Library. (If you see the .itl extensions then you should include them in the renamed files.) Start iTunes. Should all be good, bar any recent additions to or deletions from your library.
If your last previous library is from a while ago you may want to look in the iTunes folder for .tmp files. On occasion one of these may be a more recent version of your library and can be renamed in a similar fashion, editing the extension this time.