I had loaded a bunch of new games onto my R4, and tested them out, they worked fine. I turned off the DS and turned it back on again, to select a different game. Suddenly the message "Can't find _DS_MENU.DAT" appears onscreen, and I can't do anything! So then I turned off the DS and plugged in the Micro SD card to see what was up...and NOTHING appears in the files on the card. Nothing! I figured it somehow got wiped, so when I tried to drag and drop new games in, it says "insufficient space." So are all the files still there, somehow, just hiding?

How do I fix this? It was working FINE just minutes ago!