One of two things has happened: First, possible brake return spring broke or weak. If it is not against the return stop, that is what I would work on. Second, the switch is out of adjustment. Push the pedal down and push the brake switch in further, let the pedal go, it will push the switch back out, pull up on the pedal to make sure it has returned to the stop. The switches are plastic and could become loose in the mounting. Pretty cheap and easy to replace.