I bought a 40 gallon AO Smith water heater with dual elements, accidentally through the switch without the tank being completely filled up, that burned out the top element, dry shot, which will not let the tank heat, the bottom element was fine. The top element should be tested with an OHM meter if you didn't pull it out to see it physically damaged. If your top element was burned out, replace it, the top element heats the tank first then the bottom element takes over and the top element will not have power, they alternate.