Start by making sure that your thermostat is powered on and functioning properly. Try turning on your HVAC by turning on the thermostat and adjusting the temperature. Check the circuit breaker and make sure the switch is working properly if it won't turn on. Restart it if you discover that it tripped. This ought to resolve the issue.
Also, coil air returns in the HVAC system aid in filtering the air entering your home. These filters require cleaning at least twice a year. However, you might need to clean the filters more than twice a year, depending on how the system is used.