The answer to 'heat pump not working' is that there could be several common causes, including a lack of power, a thermostat issue, a refrigerant leak, or a problem with the compressor or other internal components. Symptoms may include the heat pump not turning on, not producing heat or cool air, or making unusual noises.
In many cases, the issue can be resolved through simple troubleshooting and DIY repairs, such as checking the circuit breaker, replacing the air filter, or adjusting the thermostat. However, for more complex problems or if you're not comfortable working on the system, it's best to call a professional HVAC technician to diagnose and repair the issue.