HVAC systems are complex, and a lot can go wrong. We offer 24-hour HVAC repair, and we're happy to come to take a look, but in the meantime, you can try a few simple troubleshooting tasks: Make sure the thermostat has batteries and is set to the right temperature. Ensure the unit has power and the breaker isn't tripped. In natural gas heating systems, ensure the gas is on. Check the filter and replace or clean it if it's clogged. Verify all vents are free of debris.