The answer to 'why is my heat pump not turning on' is that there could be several common causes, including a tripped circuit breaker, a malfunctioning thermostat, or issues with the compressor or other internal components. If your heat pump is not powering on, the first step is to check the circuit breaker and reset it if necessary. You should also ensure the thermostat is set to the 'heat' mode and the temperature is set higher than the current room temperature.
If those basic troubleshooting steps don't resolve the issue, the problem may be more complex and require the expertise of a professional HVAC technician. They can diagnose the specific cause, whether it's a problem with the electrical system, refrigerant levels, or other internal components, and provide the necessary repairs. Attempting to service the heat pump yourself without proper training and equipment can be dangerous, so it's generally best to call in a qualified HVAC professional.