Here is the offical Apple fix. I had the same proble, followed this and bam, alll playlists came back. Don't give up!
Re-creating the iTunes Library file
Note: After re-creating your library, any devices that you sync with iTunes (Apple TV, iPod, iPhone, iPad) will see your iTunes library as a new library and will completely resync. The next sync with such a device will take longer and may reset some options since your rebuilt library isn't familiar to the device.
1. Quit iTunes.
2. Locate your iTunes folder (this is the folder that contains the iTunes Library and iTunes Library.xml files).
Operating System Default location of iTunes Folder Mac OS X
Microsoft Windows XP
\Documents and Settings\[your username]\My Documents\My Music\
Microsoft Windows Vista
Microsoft Windows 7
\Users\[your username]\My Music\
3. Open your iTunes folder.
4. Drag the "iTunes Library.xml" or "iTunes Music Library.xml" file to the Desktop. If neither is available, learn how to add files back to the Library (because the XML file is not available, playlists and other information will not be available).
5. Drag the following file from your iTunes folder to the Trash:
Mac OS X "iTunes Library" or "iTunes Library.itl" Microsoft Windows "iTunes Library.itl"
6. Open iTunes.
Important: Do not add any content into iTunes at this point (such as by purchasing something from the iTunes Store).
7. Choose File > Library > Import Playlist.
8. Navigate to the "iTunes Library.xml" file on the Desktop.
- Mac users: Click Choose.
- Windows users: Click Open.
If your Podcasts list in iTunes is empty after following these steps, learn how to add them back into iTunes.