Music Files Take Up Memory, But Aren't There

So I was a little stupid and excited when I got my SD card, and loaded every song I had onto my player.  I loaded up the sync function in Windows Media Player, and when it loaded all my e250 could handle, I just moved on to the SD card. I loaded too much, however, and when I turned on my player, I received a “you need x MB of space” message.  But when I went in to delete files, they didn’t appear in Windows–despite it showing I had used up almost all of the memory.  I decided to mess with it later, however, and kept using it as-is.  Then I found something even worse–all my songs have disappeared from the player (neither displayed in the firmware nor in Windows Explorer) but I still get the message saying I overloaded the player with memory.  Is there a way to access these files so I can finally delete them, or at the very least delete everything and start over?  Thanks for any help!

Format it. That will erase everything so you can start over.