Turn off the AC unit
Switch off the thermostat
Turn off the circuit breaker for the AC
Wait 5 to 10 minutes
Turn the circuit breaker back on
Turn the thermostat back on
Set the thermostat to the desired temperature
Check that the unit starts running
If the unit has a reset button, press and hold it for 3 to 5 seconds
If the unit still does not work, contact a licensed HVAC technician