Take battery out and charger off. Unscrew it. Remove the bottom cover slowly. Be careful of the 2 ribbon cables when removing it, best to start lifting up from the side opposite the card slot. You will see a big black square below the card slot. That's the wifi card. pry it up (it has a removable socket) and push it back in. Most probably it's gotten a bump and got slightly disconnected. When disconnected, the dsi will perform the symptoms you describe. It could be something else, but most of the time, It's this.
Wish I helped.
Cheers