If it's been a long while since you last played it, the NAND flash chip might be corrupted. I've heard about 3DS cartridges having issues with that when left unplayed for a long period. It seems like a common issue.
If that is the issue, there's a tool out there that attempts to fix it...
It's a sham.
You heard that. Sí. It is all a sham. There is no technique practiced here. That is why they call it ro-sham-bo. It is a lie. It does not exist.
You wish you could forget the falsehoods you haven't been taught, don't you? Forget la mentira.
Sí, you want to return. Return to what...