Undelete rewritten or lost files on Garmin drive


Recently, I found out that audio files (imported music) on my Garmin c550 GPS device are impossible to delete since directory with files is untraceable. I can still choose and play music on device itself, but when connected to PC with USB cable it shows only one (main) directory and no subfolder with audio files. I think somehow I managed to rewrite them and now I need to undelete those files to be able delete them.
If my explanation makes sense, please advise.