Disappearing Music

I have a Sansa Clip 4GB (one of the old ones). There is ~1GB of music on it and playable from the player, but none is recognizable in Windows (using xplorer2 or normal explorer). The folder trees and system configuration files for the player are visible, but not the music except some that i recently added. (1 Men at Work album). What is going on? I like to consider myself fairly computer proficient (I have system and hidden files set to be viewed, etc.). The amount of free space (or lack of it) shows that the files are still there, but I have no way to access them in Windows. (I've tried all the command prompt dir tricks, by the way).

Has this happened to anyone else? Does anyone know what's causing this?