trouble with mp3 player

I have a creative muvo v100 mp3 player, and I let a friend borrow it recently, and she accidentally deleted all of my songs. When I turn my mp3 player on it says no music, and so when I tried to go back on my computer to add the songs back on their none of the songs appeared on my mp3 player, because the computer said that the songs are already on the device, when I don't see any of them on there? Could anyone tell me what may have caused this and how I can get my music back on my mp3 player.