Switch. Rheems have a reset button at the base of the external unit. The combination of high heat and dirty fins can cause it to trip. For a quick fix, you can see if the switch is tripped by pushing the button. They recommend to turn your ac setting way up so it doesn’t turn on immediately. You can hear/feel a click. But you really need to clean the external fins to make sure it doesn’t trip again. You can damage things if you just keep resetting it. Hosing it down is a start. If your dirt/gunk is super thick you may have to remove the enclosure and vacuum with a gentle brush.