Did you check the recycle bin? If the music isn’t there, is all your boyfriend’s music in his iPod? If so, then you can always download iDump. It’s a software which allows you to transfer music from your iPod onto the computer. Just do that and add all the music back. As to the cause of what might have happened… Has nobody at all touched the computer? I believe that it’s either: a. Somebody going on there and accidentally/purposely deleting it b. A virus which attacks the computer in various ways, one of them being deleting music.