We had a power failure while gas heater was on. Now we get error code HE 27. What does this mean

Error code means that you have electronic gas control valve.

Copy following link to identify gas control valve, read error code meanings for each, and download service manual relevant to each gas control, but not specifically Rheem since rheem does not post service manuals.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

The fact that water heater restarts, means that water heater is getting gas, getting sufficient air for combustion, thermister is ok, gas pressure is good, etc.
The fact that you unplug and replug water heater and it works, shows that you have power vent model.
Since water heater worked correctly before, 2 flashes with Rheem power vent model could mean that pressure switch has failed.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Before calling service technician, copy following link for ecosense tankless troubleshooting resources.
C7 is communication failure with remote, and C12 is flame failure.
Multiple error codes could mean bad circuit board, loose wire, low gas pressure, or simple communication problem with remote control.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Water heater is not 100% identified.
Unknown if this is gas or electric heater.

Problem posted under Whirlpool water heaters typically made by American water heater company.
Call Whirlpool 877-817-6750.
They will ask for model number and serial number located on label on side of tank.
Call is answered in USA by trained person who can answer specifics about that model, including warranty, age, regular maintenance, error codes, and possible solution for that specific model and serial number.
I have found these folks to be helpful.

Whirlpool manuals do not show that specific model:
http://www.whirlpoolwaterheaters.com/support/manuals.aspx

If you have the energy smart electric water heater with electronic control board, then error code explanation shows on the control board. Error 1 or a single flash, is start-up failure and requires tank to be full of water. Read page 10 of manual. Make sure water heater is full of water and water is coming full stream out of tub spout before turning on power. Also disconnect power from unit, make sure water heater is full, and then reconnect power to see if unit resets.
http://waterheatertimer.org/Whirlpool-Energy-Smart-electric.html
http://www.whirlpoolwaterheaters.com/downloads/6510306.pdf

Before calling service tech, open following links for Bosch troubleshoot resources including phone support, error codes, possible manual.
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html

No error code can mean PC board failure, or plumbing problem
http://waterheatertimer.org/pdf/No-Error-Code.pdf

Before calling service tech, open following links for Rheem troubleshoot resources including rheem chat, error codes, possible maintenance and service manual.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error codes are considered start-point for troubleshooting.
Multiple error codes can mean PC board failure.
http://waterheatertimer.org/pdf/Rheem-error-code-C7-76.pdf
http://waterheatertimer.org/pdf/Error-code-13.pdf
http://waterheatertimer.org/pdf/Tankless-maintenance.pdf

Before calling service technician, open following links for troubleshoot resources including common error codes, Bosch contact information, link to service manuals, and possible product manual
http://waterheatertimer.org/Troubleshoot-Bosch-Tankless-water-heater.html
http://www.boschhotwater.com/HelpfulResources/TechnicalSupport/BoschAquaStarModels/2400ESTechSupport/Troubleshooting/tabid/1295/Default.aspx

Call Bosch 800-642-3111
Multiple error codes can mean PC board failure.
AF might be A9, but not sure.
EA is 'no flame ionization detected with water flow'

1. Verify that all manual gas shut off valves are open.
   
2. Check gas type.
   
3. Reset error code and open a water tap to cycle the heater in an effort to purge air on new installations. Cycling hot water tap on and off multiple times may be necessary. If heater still faults with EA error code, have a licensed gas technician properly purge air out of the gas line leading to the water heater.
   
4. Check three wire connections to ignition group on the lower front of the heat exchanger are secure.
   
5. Check gas pressure.
   
6. Check venting specifications are met. Improper venting may cause premature failure of the flame sensor rod.
   
7. Check that the minimum power fan speed has been adjusted to the proper value.
   
8. Observe inside the viewing window of the heat exchanger when a hot water tap is opened. Sparking should be followed by a steady blue flame. If flame is unstable or yellow with proper gas pressure.

Before calling service technician, open following link for Rheem troubleshooting resources including Rheem chat:
Rheem will suggest you catch up on required yearly maintenance which would cover most causes for common error codes, and cover cause of no error code including plumbing problem or loose wire or PC board failure. Combination of problems can also cause failure and these issues would be covered by general maintenance.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html
Solve most tankless problems same day for $350
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Open following link for useful troubleshooting information:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

Error code 11 is 'No ignition'
http://waterheatertimer.org/pdf/Paloma-Tankless-Error-Codes.pdf

Ensure you have gas to the appliance.
Ensure gas type and pressure are correct.
Bleed all air from gas lines.
Ensure gas line, meter, and regulator are sized properly.
Ensure appliance is properly grounded.
Check gas solenoid valves for open or short circuits.
Ensure igniter is operational.
Check igniter wiring harness for damage.

Error code 12 is 'Flame Failure'
Ensure you have gas to the appliance.
Ensure gas type and pressure are correct.
Bleed all air from gas lines.
Ensure flame rod wire is connected.
Check flame rod for carbon build-up.
Ensure gas line, meter, and regulator are sized properly.
Ensure appliance is properly grounded.
Check gas solenoid valves for open or short circuits.
Check power supply for loose connections.
Check power supply for proper voltage and voltage drops.
Disconnect remote control.
Disconnect and re-connect all wiring harnesses on unit and PC board.
Ensure proper venting material was installed.
Ensure condensation collar was installed properly.
Ensure maximum vertical vent length does not exceed allowable limits.
Ensure maximimum horizontal vent length does not exceed allowable limits.

Range of causes are part failure, gas pressure loss, air in gas line, improper installation of gas line or vent, 120V power to unit is disrupted or unit not grounded, circuit problems, venting problems, remote control, wiring to PC board, board failure, carbon build-up on flame rod, short circuits on gas valve solenoids.
The fact that you have error code tends to eliminate your plumbing as culprit, the error code does not mention air supply, otherwise it looks like every possible cause is on the table.

Add a comment with your experience so other with same-similar problem can benefit.

Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Some error codes cannot be reset...Example would be P/S failure on the PCB. Any other error, if you remove power for approx. 2 min - error should clear. - if not you need to contact Paloma/Rheem customer (800-432-8373) support

I guess this a gas heater.You need to look at the manual to find out what code 11 is.It could be misfired or tempture to high.Give me a litte more infor.thanks