Rheem 83xr52-2 50 gal residential water heater. Water temp was low. I cut power at electric panel, removed upper panel on water heater, turned up thermostat, replaced panel and switched power back on at electric panel. Now water heater won't turn on at all. Help.

You can open the top up again. I would not turn the temp higher than 130 degrees, maximum. However, there should be above the thermostat a red button. This is where the power wires connect to the thermostats and heating elements. Sometimes this button needs to be reset, press it and discern if you hear a click sound. This means that the high temperature safety switch has tripped open and you had to manually "reset" it. There are two thermostats by the way. The water heater usually works this way. There are two thermostats and two heaters. The upper tank heater turns on first and it heats the top half of the tank. Then when the thermostat above is satisfied it then allows the bottom heater to come on and heat the bottom half of the tank. You might want to set the bottom heater thermostat as well. This is made this way because it is actually faster and more efficient to heat the top half first and the bottom half secondly. Try this first with the reset button. We could check voltages, but that requires a level of knowledge and safety that I am not sure that you are comfortable with. Pictures of the upper and lower thermostats and connections will help if the "reset" does not do the trick. By the way, not meaning or implying anything, but did you move the thermostat in the right direction? Sometimes the dials can be confusing.

Electric water heater is completely repairable by homeowner.

What to do right now:
Check circuit breaker
Push in red reset button on upper thermostat and see if that restores some hot water.
Put insulation and cover back so thermostat does not receive cooler air and misread tank temperature which can cause overheated tank that causes reset button to trip.
Turn thermostat back to where it was.
That way you have some hot water until problem is fixed tomorrow.

Break down problem:
You were short of hot water.
That says electricity to water heater is good and upper element is working.
And it says original problem is one of 4 things.

1) Loose wire going to lower part of water heater. Check parts for signs of heat damage at screw terminals, especially lower element.
Burned lower element means lower element probably bad.
http://waterheatertimer.org/Replace-element-and-clean-sediment-out-of-electric-water-heater.html

2) Bad upper thermostat ***** This is best suspect since water heater stopped completely after temperature adjustment.
3) Bad lower thermostat. I usually replace both thermostats at same time.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

4) Bad lower element. Test element using inexpensive multimeter from Home Depot electric department.
http://waterheatertimer.org/How-to-test-water-heater-element.html

Here are repair steps:
Buy generic upper and lower thermostat and replace both thermostats, and if problem persists, then replace lower element.

If you want to test water heater parts before replacing anything:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Upper thermostats are interchangeable
Upper thermostat for 2-element tank has 3 screws on right side of thermostat