Many times the float ball becomes stuck alongside the tank. Alot of floats are on a water proof cord which floats up when the water level rises, if the pump is in the wrong position, it can become stuck in the up or down position. Check this first. If that is free, you may have to either replace the float switch or the pump itself. Check the age of the pump, most pumps have a 5 to 10 year life span. If your close to that age, it may be advantageous to replace it at this time.