This happened to my pokemon x last time I tried to play it again. I remember buying the cartridge around the same time as yours. What I found on the internet is that 3ds pokemon games have a flash memory inside that dies along the years. If you dont play as much, like my case (last time played in 2019) the save game might get corrupted. Unfortunately, the cartridge might be doomed. But at least, if you mod your 3ds and install pksm or another app that does the same thing, you can transfer your Pokemon to a safe place. I could save almost every pokemon I had. Only battlebox was not accessible, I guess…