Storage water tank overflows and the motor won't shut off. What is the problem?

You didn't give us much info to go on, but there is probably an electronic float valve in the storage tank that is supposed to shut off the pump when the tank is near full. Must be either stuck or malfunctioning.

If a ball cock is your fill valve pull up on the ball and see if it shuts off. If it does not then you have a faulty ball cock that must be replaced. Good luck, Tom