The answer to 'heat pump not turning on' is that there are several common causes, including a tripped circuit breaker, a faulty thermostat, issues with the outdoor unit, or problems with the refrigerant lines. Symptoms may include the heat pump not powering on at all, the outdoor unit not running, or the system blowing cool air when you expect it to be heating.
In many cases, the solution can be as simple as resetting a tripped circuit breaker or replacing the batteries in a thermostat. However, more complex issues like refrigerant leaks or compressor failures will likely require the expertise of a professional HVAC technician. It's generally best to call in a pro if you're not comfortable troubleshooting the issue yourself.