maybe the path to your library file got changed search for files name 'iTunes library.itl' and 'itunes music library.xml' then in itunes go to edit | preferences | advanced. in the top part of the window, under 'itunes music folder location' there will be a path to your library. if this isn't the same as the path to the itl and xml files, hit the 'change' button and browse to the correct path. try that and then report back if it didn't help at all