Check the settings on your thermostat

 Check to make sure that your thermostat is set in the "COOL" or "AUTO" position. 

 Make sure the temperature control selector is set below room temperature. 

 Check the FAN switch should be set at ON for continuous blower operation or AUTO if you want the blower to function only while the unit is operating.

 Ensure that your outdoor air conditioning (condensing unit) is running

 Check the circuit breakers in the circuit breaker box (or electrical panel), most likely mounted to an outside wall in the back of the house. Are they in the "On" position? 

 Check the outdoor unit "disconnect switch" to make sure it is in the "ON" position. The disconnect switch is located near the outdoor unit. (Typically a grey 8" wide x 16" high x 4" deep box mounted to the wall).

Ensure that the blower motor in your furnace is running

 If the thermostat is in the "cool" position, the furnace blower should be running.  If not, check to make sure the on / off switch at the furnace is in the "ON" position

Be sure that you have changed your filter in the furnace recently

 An extremely blocked filter can cause your outdoor air conditioner unit to shut down due to lack of proper airflow. 

Your filter should be changed on a regular basis (depending on the filter type).

Check all return air grilles to make sure they are not blocked by furniture

 Check all supply air registers to make sure they are open and blowing air. (The return air grilles are normally located on your walls and are wide and flat).