Turn the thermostat to the "Off" position. Wait a few minutes, then turn the thermostat either to "Heat" or "Cool."
Set the thermostat to the desired temperature. Wait a few minutes to listen for the sound of the system turning on.
Go to the circuit breaker box if you do not feel or hear airflow. Locate the circuit for the HVAC system. If it is tripped, move it back to the "On" position. If it is already in the "On" position, flip it to the "Off" position and then back to the "On" position.
Turn the thermostat off and then on again. Set the temperature to complete the reset process.