DISCLAIMER: With the exception of the thermostat and the disposable return air filter, your air conditioning and heating equipment is best handled by a trained technician. Mishandling your air conditioning and heating equipment and your breakers may result in damage to the cooling and heating system and/or personal injury (including death by electrocution). Use caution and proceed at your own risk.
Is your thermostat blank or is it displaying the words "Change Battery"?
Most of the newer, electronic thermostats use a battery. Unclip the thermostat from the wall and look underneath it. If there is a battery, replace it with a fresh one.
Has anyone been in the attic lately?
Many attic units have a power switch, identical to a light switch, which turns power on and off to the unit. People sometimes inadvertantly turn off this switch on their way out of the attic.
Is there water standing in the drain pan in your attic?
Many cooling systems now have a float switch in the drain pan which shuts off the air conditioner when the condensation produced by the unit isn't draining properly. The purpose of the float switch is to prevent water from overflowing the drain pan and damaging the ceiling or walls. If water has backed up in your drain pan, call a service technician to investigate and treat the cause of the standing water. If you bail or wet-vac all of the water out of the pan, the float switch may allow the air conditioner to run while you are waiting for the technician, but keep a close eye on the drain pan and the attic unit. Some problems (dirty coils, burned contacts, low refrigerant) will cause the coil in the attic to become caked in ice then defrost at a faster rate than the drain lines can carry away the melted ice. When this happens, there is a risk that the ice may extend beyond the drain pan and bypass it, causing water damage when it melts.
Have you changed your air filter recently?
A dirty filter will restrict air flow 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?
If the breaker is tripped, turn off the air conditioner at the thermostat, reset the breaker, then turn the air conditioner back on. Sometimes a power surge or brown out will cause the breaker to trip. If you air conditioner is regularly tripping the breaker, your equipment needs to be checked by a technician.
Is anything obstructing air flow to the outside (condenser) unit?
Overgrown vegetation or anything else obstructing air flow around the condenser can compromise cooling. Likewise any debris inside the condenser (ant mounds and grass clippings are common culprits) can keep it from functioning properly. Be sure to turn off the breaker before removing any debris from the condenser.