If you have a multimeter it will be easy to isolate the problem.
1) Check AC hot water tank breaker not tripped
2) Check for presence of 120 at battery side pole of the on/off switch; if none have wiring problem to water heater which is not likely
3) Test for presence of 120 at the downstream pole of the on/off switch, if none replace the on/off switch
4) Test for presence of 120 at the downstream side of the AC thermostat, if none press reset button, if still none replace thermostat
5) Test for 120 across the posts of the water heater element, if it was at the thermostat it should be here or you have a bad wire/connection at the thermostat/heater element.
6) Turn off the AC breaker for the Hot Water tank (as you can't use your on/off switch by the tank) and test for 10 ohms resistence across the posts of the water heater elelemt, if open circuit replace water heater element
7) Make sure water on, breaker on and all protective rubber covers back in place and go have a nice hot shower