What type of water heater?
Atwood is notorious for the ground wire/screw (connected to the exhaust flume) coming loose or rusted and losing continuity. If tight, loosen and clean before reinstall. I also had either the thermostat or ECU go bad on mine - cutting power to the heating element. Usually a $10 - 15 purchase and easy to install. Try replacing if the ground didn't remedy. Also check the thermo-fuse before replacing the board and/or the heating element. Note: the 110V thermostat/ECU are usually on the back side of the tank!
Suburban units a little different in that the thermostat and ECU are resettable. Push the button under the rubber cover to ensure they haven't tripped. The features of the thermostat and ECU are the same on both units - if bad they cut the power to the heat element.
NRVIA Certified RV Inspector #523
2003 Monaco LaPalma 36DBD - 2013 Honda Fit