Problems at the Thermostat

• Double check your thermostat before getting more involved. Ensure that the thermostat is set to cool and that the temperature is lower than the current room temperature.
  
  

Electrical Problems

• Ensure that your air conditioner is getting electricity. Start with your circuit or fuse box and then check the unit itself. Air conditioners, for example, can shut themselves off if they detect condensation overflow that has a chance of reaching electrical components in order to prevent worse damage.
  
  

Filter Problems

• Check the air filter: if it is covered with dirt or other debris, clean it. At the same time, check the outdoor compressor for damage or blockage. If air can't move easily into and through the system, all of its operation will be impaired.
  
  

Overheating

• Your outdoor compressor may be overheating. Generally, this is a sign that professional A/C work is needed.