If you had a power outage and your Rheem tankless water heater, UMC-117 will not turn back on, try closing, both the cold and hot water valves at the bottom. The unit will not turn back on if it senses, water pressure in the system for some reason. You also cannot just close the main water valve. It has to be both the hot and cold water at the bottom of the unit.
Once you close the valves press the power button and it should turn on right away. Then remember to open both valves back up.
I don’t know why this is not listed on any of the Rheem documentation. I had to contact Rheem tech-support, and they literally fixed it in less than five seconds. 
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