This is probably the most common Summer-Time Complaint!
First things first. Please check all of the obvious things before calling for a service-call.
Is the thermostat on "cool" and turned down below room temperature?
Is the Power on to the indoor unit? Did you check the breaker?
Is the Power on to the outdoor unit? Did you check the breaker?
If yes to these, is the outdoor unit running?
If no, see if the outdoor unit has a red reset button (most Rheem equipment does, located near refrigerant lines), and push it.
If that didn't work, see if you have a condensate pump at the indoor unit. If you do, some pumps have a safety switch that shuts off the outdoor unit if the pump overfills and fails to pump. So check the pump to see if it is working.
If that wasn't the problem, back outside at the unit there usually is a disconnect box mounted on the wall. Some of these have fuses or a breaker inside. If there is a breaker, make sure it didn't trip. If there are fuses, they should be checked. Caution: High Voltage, danger of electrocution. Please do not attempt this unless you are familiar with working with electricity.
After having checked all of these things and your unit still isn't running, now you can call for service.
For more possible causes as to why your outdoor unit isn't running, please see:
For more possible causes as to why your outdoor unit isn't running, please see:
Outdoor Unit Won't Come On