The air conditioner was working, but after a short stop and after starting the engine up again, I only got hot air blowing out of the vents. The rear air unit showed ''Power Off'' when I tried to see if it was working in the rear. Help! BobJ

There are 2 problems:
1-
To keep the AC on there are a lots of feed back to Engine control unit from: Refrigerant press, Engine temp sensor, Evaporator Temp sensor, Cabin Temperature sensor, Compressor Speed sensor, Engine Speed Sensor. All of these need to be in the spec range for the Compressor clutch to engage then AC could function properly.

2-
The hot air blowing out since your blower is still working, but divert valve problem: The divert Flap vent instead of directing air through the evaporator path, It is directing the air through the heater core.
Unless you have equipment to check all of that and time consuming, you might want to have a hsop to estimate first ro pay an diagnostice and decide to fix later after decide the problem could be tackled by your expertise. I'd check back to see how your progress on this matter. Good luck in good findings.