help required !!!! lost all tunes !!!!

Hi guys a friend of mine has lost all his tunes on his ipod and has asked me to see if they can be retrieved here's what happened!...

he set up his ipod to his girlfriends computer and used it to load his tunes on with that computer, but he plugged his ipod into his own computer and it came up with the message "this ipod is registered to another PC" and all his tunes were missing but since then he has plugged it back into the original computer he used it on and all his tunes are missing from there too !!

is there anyway way he can get back the tunes ?? i mean they cant just disappear... can they ?!?

Thanks for the help