Did you use an Action Replay on it or play the game on a different ds that has used an action replay? If so, that's pretty reasonable. It's happened to me several times as well.
If you want a solution, here you go: You can try sending it in to Nintendo where they can fix it. You should get your game back in a couple days.
Pros: Fixes the game, prevents freezing
Cons: Restarts your game. Yeah I was pretty sad when that happened to me :/
That's what I would recommend for you to do to stop the freezing.
Hope I helped!