Most of the problems with a Rheem gas water heater will be related to the FVIR lockout. With most models your first step should be to press the TCO thermal reset button which is located on the front of the combustion door.
Clean the Intake
The next step should be to clean the intake of the water heater. A clogged intake can cause the thermal button to trip. A shop vac with a custom extension or even compressed air can be used to clean the flame arrestor screen (intake). In order to access the intake you will usually need to remove the combustion door.
TRD Glass Vial
The above steps will not work for every type of Rheem water heater or for every situation. Many of the Rheem water heaters have a Vial that is located inside of the combustion chamber. If the vial has burst, you will need to wait for a new vial to be shipped. You should also clean out the combustion chamber and pilot assembly. If the pilot is yellow instead of blue there is too much corrosion on the pilot assembly.
Thermocouple or Gas Valve
The next step in troubleshooting should be to test the thermocouple. When attempting to light the water, the flame will usually go out after releasing the control knob when the thermocouple has failed. If the flame continues to burn after releasing while in the PILOT position but shuts down when in the ON position the FVIR may have tripped the system. If the thermocouple shows a good reading you may need to replace the gas valve and FV sensor.