There is several things that could be bad. First, off you need to see at what point there is 120 volt AC power to. You need to take a multitester and start at the element, then work backwards...... or you can start at the power outlet for the water heater and work forward. Many times the water heater isn't even plugged in at an outlet behind the heater.
If working forward, then you will have power from the outlet and then check the little black switch. You will need to make sure there is power on both sides of the switch. You test each side of the switch by placing the hot probe on one terminal then the negative probe on a ground somewhere. Don't just touch the two probes to each terminal, as this won't show you power since it;s in the same one line.
If you have power on both sides of the switch, when testing the switch, then you move on to the hi-limit ECO thermostats. These are underneath the rubber covers you are pressing to reset. These are high temp emergency cut off and thermostats built into one housing. You test the same as the black switch, since it's all on the same line.
You have two of these, one for the 120 volt and one for the 12 volt propane side The 120 volt is the one on the left if I remember. If you have 120 volts on the top terminal, then move to the bottom terminal. few things to remember right here though. If the water is hot inside the tank, then the thermostat shuts off the power right here, so you won't have any power flowing to the bottom terminal if the thermostat is working correct. You need to make sure the water is cool in the tank to test. If you have 120 volt power coming out the bottom terminal then:
You move to the heating element. Here again you test for 120 volt power at it.
2011 Flagstaff 831 RLBSS
I used to be addicted to time travel, but that's all in the past now.