Follow the electrical leads from the appropriate circulator to a point where you can measure and see if proper voltage is being applied. You can check the voltage vs a working zone pump leads as a refereence. If you have voltage, tap the pump light to moderately with a suitable non marring hammer or large screw driver handle. It may be just hung up and the jarring should free it. You could also have an air lock at the circulator if there is a leak in that leg. If I remember right you can apply a low psi 5-7lbs through the overflow pipe and it is enough to move the air lock and start the flow if that is the problem. That tip came from Roger Berke. You may want to join his forum to get the best info on this system.
Later Ed
