Is the thermostat set below the ambient temperature? The unit will only kick on when the temperature rises above the thermostat setting. On humid days, the air can feel sticky and warm even when the temperature is in the mid seventies.
Is the thermostat set to cool? If not, adjust the thermostat.
Has anyone been in the attic lately? Many attics have a power switch, like a light switch, which turns power on and off to the attic unit. People sometimes inadvertently turn off this switch on their way out of the attic.
Have you changed your air filter recently? A dirty filter will restrict airflow and compromise cooling. Replace your disposable filter if you haven’t done so recently. If you have an electrostatic filter, take it outside and thoroughly clean it.
Have you checked the circuit breaker marked AC? If the breaker is tripped, reset it. Note: If you air conditioner is regularly tripping the breaker, your unit probably needs to be serviced.
Is anything obstructing air flow to the outside unit? Overgrown vegetation or anything else obstructing air flow around the condenser can compromise cooling. Likewise, any debris inside the condenser (such as grass clippings or ant mounds) can keep it from functioning properly. Be sure to turn off the breaker before removing any debris from the condenser and then reset the breaker.
Is the outside unit (condenser) icing over? If there is ice coating part of your outdoor system, there is a problem with your system. Common causes of condensers icing over include refrigerant leaks and dirty coils. Turn off your air conditioner so that the unit can completely defrost. An air conditioning technician will not be able to properly troubleshoot or service your system while the condenser is iced over.
Is there anything obstructing your return air grilles? Make sure no furniture or other items are blocking you return air grilles. Furniture and items are supposed to be at least four inches away for adequate air supply.
Is the outdoor unit running? Check your circuit breaker inside and make sure the disconnect switch outside is set to the “ON” position. The disconnect switch is located near the outdoor unit. Normally it is a gray box mounted to the wall.
If your unit is a heat pump, it is normal during the winter for the outdoor unit to occasionally be covered with a white frost. Heat pumps have a defrost mode that will de-ice the coil. If a thick layer of ice develops, you have a problem that a heating and air professional will need to determine. Turn your unit off until serviced to prevent further problems.