Whoa. I remember that my friend gave me his Memory Card for the GCN to see what was wrong with his files. It turned out the same exact thing happened. Oddly enough, the same thing happened to me on SM64 DS. First, my younger sister deletes my COMPLETE Save File with EVERYTHING on it, then when I'm just about close to doing it again, I open up the game and get a message saying, "File seems corrupted." And all of my data was erased. =( So unfortunately, you have no choice but to delete the data and restart the game. I'm sorry that this happened.