(1) Did you change your Home directory's name? If you did, check your users folder if there is more than two folders there (ideally there should only be two if you don't have multiple user accounts, the one you are currently using and a shared folder), check out the contents of the music folder in each of those folders. Changing the name of the home directory confuses iTunes as to the location of the files, that's why it won't show the songs in iTunes. If you just accidentally deleted or moved the files out of the music folder, the track info should still be in iTunes but it won't play because the file obviously isn't there.
Note that if you are manually managing the file location of your MP3s try to remember if you moved any folders containing your songs recently.
(2) Have you tried deleting the preference file of iTunes?
(3) Have you tried searching for your *.mp3 files? If you find them, You just have to drag everything back to iTunes.
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