what disappear was the list of music on your itunes but the folder of the mp3 and mp4 still on itunes folder or whatever folder you have your music on you just need to drag them back to itunes the real problem is that if you did some changes to the name of the song on itunes it might be change back so you have to do it again

for example this is what happened to me
let say i add cream to the itune and it looks like this
Cream Wu-Tang Enter the Wu-Tang (36 Chambers)

then i change it to
Wu Tang Clan - CREAM Wu Tang Clan Enter The Wu Tang (36 Chambers) Hip Hop

when it happen to me and i dragged them back the name of the song return with the same name when i first added them
i spend weeks fixing the names of the songs but the reason it happen to me was cuz i change from windows to mac so that was a bitch