There are many different spa problems that can result in the heater not working. As always, be sure you are qualified to work on electrical appliances before attempting any hot tub repair or call a qualified service tech. Turn power off to diagnose and isolate problem to make sure you have identified the problem component. In this case even though the heater was not working, the problem turned out to be something else.
First we will remove the solid state cover (4 or 6 screws that attach the cover to the spa pack).
We start from the temperature sensor and determine that there is 50 ohms of resistance. This should be 75,000 ohms resistance, so the problem was with the temperature sensor set and this one needs to be replaced. This is a $92 part along with $80 in labor. See how fast the problem was figured out. So the pump would run and there was no heat, the heater was not the problem but rather the sensor.
Note this service is being done with the spa power OFF.