This has happened to me a couple times in my Pokemon Yellow and Pokemon Emerald. I think it could be that the game card slightly moved out of place by accident, which can result in the game freezing on the original DS (I don't know if it does the same on the DSi or 3DS. I haven't tested that out before). As well, there may be slight damage on the card, but if it saves perfectly fine the next bunch of times, I could possibly rule out any damage. I know for my Emerald, I would blow on the inside if it froze and it would work again, so maybe doing that to remove any possible dust off the game might help?