The answer to 'why is my heat pump not turning on' is that there could be several common causes, such as a tripped circuit breaker, a thermostat issue, a refrigerant leak, or a problem with the compressor or other internal components. Some common symptoms include the system not responding to the thermostat, strange noises, or a lack of warm air circulation.
If your heat pump is not turning on, the first thing to check is the circuit breaker. Make sure it has not tripped and, if so, reset it. You should also check the thermostat to ensure it is set to 'heat' and the temperature is set higher than the current room temperature. If those basic checks don't resolve the issue, it's best to call a professional HVAC technician to diagnose and repair the problem, as there could be more complex electrical or mechanical issues that require their expertise.