Yes, the control module for the water heater is taped to the foam insulation on the back of the tank. Not a very good tape job, either. If it comes loose, the plug to the module can be dislodged. The other thing to check is the Hi-Limit switch on the outside face of the water heater. There are two black rubber "buttons" there. One is for 12V operation, the other for 120V. If it somehow got tripped, you need to press it (press both, won't hurt). If it clicks, then it had tripped for some reason.
If you somehow ran the water heater on electric without enough water in it, you may also have fried the heating element. I'm not sure if that would result in an incomplete circuit and the switch not illuminating, though. Maybe.