Clip lost all contents of the Music folder

I turned on my Clip last night and Playlists showed nothing. I checked the USB mode and it had gone back to Auto Detect (in which it seems to default to MTP). I set it back to MSC (which it had been) and still no playlists. I connected it to the computer and the Music folder was empty. Any ideas about what could have happened?

What had happened was that the contents disappeared from being accessable but still took up space on the player, thus reducing the capacity. I just formatted the player and recopied the music. That worked. What actually happened and why is still a mystery.