Check to make sure that your thermostat is set in the "cool" position. And the ON-OFF switch is on the "On" position.
Check the circuit breakers in the circuit breaker box (or electrical panel). 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 small electrical box mounted to the wall within reach of the unit).
Ensure that the blower motor in your indoor unit is running.
Make sure your filters are clean.