First, check your circuit breakers for a tripped switch. Note: Changing the thermostat quickly can cause a disruption. Allow 15 minutes for the compressor to automatically reset itself. Next, make sure the blower compartment door is closed. The unit will not come on if the door is open. Lastly, shrubbery around the outside unit can also interfere with proper operation. Make sure there is 1-2 feet of air around the outside unit. If the air conditioner still does not engage, contact your HVAC technician.