This problem is typically from a defected or broken XML file in the Music library folder. In order to fix this, go to: ~/Music/iTunes/iTunes Library.xml
Either delete this file (Move it the trash) or move it to the desktop. Either way, iTunes will have to recreate it. First, of course, QUIT iTUNES!
If an error occurs (which is likely, because it is a broken file) and you cannot move or delete it, try deleting it with the Terminal.
Be very careful not to delete the wrong file. The Terminal doesn't simply move the file to the trash, it DELETES IT. Here is what you need to do for this:
Open up a new Shell and type: rm -rf
Then, to eliminate confusion or accidents, just drag the file to the shell and the exact path will appear after it.
Voila! It should be deleted.
Then, go ahead and open up iTunes. Rename a song or something and the iTunes Library will recreate the iTunes Library.xml file.
This works the same for just about any iLife application! If there is a problem accessing anything in the Media Browser, find the Library.xml file and delete it, then force the application to recreate a fresh, unbroken copy.
Hope this helps!
Here are a few links that should help you out: Deleting a File with Terminal
iLife: Cannot See iPhoto Files