If your heat pump water heater doesn’t go on at all, there is a problem with the thermostat or the unit receiving power. Do the following:
to the proper setting (Heat if you’re calling for heat and the temperature you want the room to be). If the thermostat was replaced recently, the new thermostat may either be the wrong type (it must be specifically a heat pump thermostat) or it may have been wired improperly. A mistakenly wired thermostat can fry electronic components, preventing the heat pump from working properly.
Turn the circuit breaker all the way off and then flip it back to ON.
The two circuit breakers that protect the electrical circuits providing power to the air handler and heat pump condenser may have tripped. Check both the main electrical panel and any subpanels that supply power to the unit. If either circuit breaker has tripped, reset it by flipping it to OFF and then to ON. If the circuit breaker trips again, there is probably a short in the electrical system providing power to the furnace. Call an electrical contractor.
to a power switch, either on the wall near the unit or inside the air handler cabinet, be sure it is turned on (many don’t have switches). If it is turned off, turn it on and wait a few minutes for the air handler to engage.
that provide supplemental heat (most do), the circuit breakers or fuses that protect the heating elements, which are usually located in the air handler cabinet, may have tripped or blown.
Opening up the cabinet of a heat pump and working with electrical parts can be dangerous for the inexperienced. If you don’t have the necessary knowledge and skills, call a heating repair technician.