The answer to 'HVAC system not working' is that there are several common causes, including issues with the thermostat, air filters, electrical components, or refrigerant levels. Symptoms may include the system not turning on, poor airflow, or inconsistent temperatures.
Some basic troubleshooting steps homeowners can try include checking the thermostat settings, replacing the air filter, and inspecting the circuit breakers. However, for more complex issues or if you're not comfortable with DIY repairs, it's best to call in a professional HVAC technician to diagnose and fix the problem.