corrupted file

what could be the possible reasons why would a rom loses it's save file?? because my friend played this one game on his r4 then after some few weeks all of his efforts to finish the game was wasted TT_TT....

That's not the only reason why it would happen, though. I have Super Mario 64 DS, the actual game, and it still erases my file every time I get between 60-80 stars (Gasp, I'm on 70 this time!)