Normal game cards would'nt normally freeze, so cheats are one of the only options. Cheats such as "have all items" can be dangerous as you may be supposed to have the item at a certain point of the game, and the game gets confused as to why you have the item so early, and it freezes up. "Walk through walls" has a similar effect.
If you are sure you haven't used any cheats, then another possibility is that if you have opened up the game card to possibly fix something, this can bugger up a game as well.
Wireless communications/interent and/or Nintendo Wi-Fi can also mean trouble, but that's only if you are connected to someone who uses cheats. Whenever you are underground, you DS is automatically searching for other underground users, so it can easily pick up any abnormal data from someone else.
Dirty game cards or dirt in the system also cause trouble, and blowing only makes it worse (causes moisture, which can't be good). Wipe it gently with a cloth if you find any forign material on the game card. The instruction booklet the came with your DS should probably provide further details (Now I'm starting to sound like Nintendo :|)
The reason can't be resolved without pretty specific details provided, but above ^^ are some of the likely causes.
So yeah, sorry bout' your game, and I hope this helps :D