~/Library/Preferences/com.apple.iTunes.plist and nearby files as a start
(You have to hold down the option when using the Go menu to get to the Library)
No quarantees of course, but borked pref is always a good place to start.
If that doesn't work, trash ~/Music/iTunes Music Library.xml (If I remember rightly, you will be prompted to create a new file at launch)
Also, don't forget to use the feedback agent.