Can't play music

It looks like the songs have been transfered to my MP3 player, but when I go to listen to them it says “no song” when I go into the music section…any ideas why this is happening? I’ve checked under playlists, songs, albums, etc and nothing there, but when I hook it back up to the computer, it shows that they are there…wierd, and frustrating. Any help would be appreciated.

Thanks!

Are the songs .mp3 or another supported format?