Before you adjust your brakes, I would make sure that the wheel is centered in the frame. Most of the time, brakes don't just go out of adjustment. Usually it's either the wheel is cocked to one side or the other or a broken spring (for cantis or v-brakes).
If it's a road bike, the brake itself can get knocked to the side as well. That you can usually just grab the brake and recenter it by hand.
