Thermostat Settings Wrong
Sometimes you can underestimate the temperature in the house. The house may be much warmer than you think it is. If you have a digital readout, the thermostat should tell you how hot it is in the room at the moment. Set the desired temperature lower than the current temperature. If the current temperature is 77 degrees and you have the thermostat set to 78 degrees, it’s not coming on until the mercury climbs a bit. Try turning your thermostat down several degrees to see if it kicks on within a minute or two.
Power Turned off
One factor that is certain to keep your air conditioner from running is a lack of electricity to the unit. If a circuit breaker has been tripped, you’ll need to turn it back to the “on” position to get your system up and running again. A power switch is also located on the air handler, usually found in a closet somewhere in the home that may have been bumped to the "off" position. Another service shutoff switch is located on the exterior of the house near the outside portion of the central unit. It must be turned on as well. Any of these will keep the current from reaching the system.
Broken or Burnt Wiring
Broken, frayed or burnt wiring can prevent proper function of an air conditioner. It can also be the source of an electrical smell and potentially be a fire hazard. Shut off all power to the system if you find faulty wiring, and get a professional electrician to repair the wires before starting the system back up.
Lock out or Reset
Overheating caused by clogged air filters or other factors can cause an air conditioner to shut down and even go into a lockout mode. This safety feature makes turning the system back on possible until the reset button on the outside unit is pressed back in. It may also require the breaker to the system to be turned off for a minute and then turned back on for proper functioning.