Did it every hit/fall on anything? Did you buy it from GameStop? Perhaps it's the effects of the previous user's hacking... If not, can you show us a picture of the back of the cartridge (maybe it's scratched or something)? I can't think of any other reasons why this would happen to an un-hacked cartridge. Also, if you have Pokémon Bank you can try accessing the PC Boxes through there. This is sometimes possible and making a change tends to correct your save (or will at least salvage any Pokémon not in the active party or day care).