Your music library file has all the songs that you see on your iPod, but your actual music files have become disassociated from the library file. Meaning when you try to play a song in your library, it looks for that song at a certain spot on the hard drive for the file. It doesn't find the file, so it skips to the next song. I would restore your iPod, and reload your music. Its a glitch in the software, the restore should fix it. My iPod did this once too.