Does your toilet continue to fill long after it should have stopped? A faulty flapper or unhooked chain can keep the water flow going indefinitely. Those are simple fixes, and once you check things out in the tank and make any necessary adjustments the water should stop. If so, great! Give yourself a pat on the back.
On the other hand, if it is still running and you are not partial to going for a swim on the bathroom floor, or mopping up the basement, your next step is to shut off the water to the toilet until you can get a plumber on site. Simply turn the valve behind the toilet until the water supply stops.