First, there is no such file as "itunes.itl". There is a "library.itl" (you need to be precise when asking questions here or it's garbage in - garbage out).
Which version of OSX on your Mac?
Which version of iTunes on your Mac?
Have you used your iTunes collection with a different computer?
Have you done any other system updates, or software updates on your computer recently? iTunes won't suddenly do this unless your library file is corrupt. It will irreversibly be updated by using it with a newer iTunes version. Are you sure you don't have two copies of iTunes on you computer, an old one and a new one? Search for iTunes application(s) with Spotlight.