The answer to 'why is my HVAC system not turning on' is that there could be several common causes, such as a tripped circuit breaker, a thermostat issue, or a problem with the HVAC unit itself. The symptoms may include the system not powering on at all, the fan not running, or the unit not producing hot or cold air as expected.
Some potential solutions you can try yourself include checking the circuit breaker, replacing the air filter, and verifying the thermostat settings. However, if the issue persists or you're not comfortable troubleshooting the system, it's best to call a professional HVAC technician to diagnose and repair the problem properly.