Many playlists are empty after update (10.5.1)

I recently updated my iTunes to 10.5.1. After the update, I noticed that many of my pre-exisiting playlists (normal, not smart lists) were completely empty. The playlist name will still appear in the list of playlists, but now it simply won't "hold" any content. The songs themselves are still in my library, but I now have no record of what songs were in what playlists. So, for example, a playlist called "workout" had 23 songs before the update. Now, the playlist is entirely empty. Is there anyway to fix this? Or at least get some kind of files that lists which songs were in which list?

Many thanks in advance