Probaly a blown thermal fuse located as a plug in on the right side of the electronic module behind the panel. Part # AP3178588 This blows sometimes if the water temperature is well below 110 degrees and/ or there is a power surge at a bad time in the cycle.
It can also blow on a heater/thermostat failure. Unit should be checked with an amprobe, combined heater/motor current typically less than 9 amps.