I had that problem too, my music was stored on an external hard drive if that makes any difference, though i cannot see why.
I have had trouble after updating itunes so leave it alone nowadays.
I had to delete all my music files from my music/itunes etc and also delete from the itunes library window, then reinstall all the music fom my back up disks i had made.
Since i did that it has been fine, except i no longer have the files on my external drive as it gave up the ghost, maybe this had some bearing I do not know. I am getting a new external soon and will put it all on there and see if the problem returns