Set the voltmeter on DC Volts (if it has a range use 0-25 volts).
Provided the power center fuse tests good.
With the water heater wall switch "on", put the red lead on the top red wire of the right thermostat next to the reset button. The black probe should be on the metal frame. There should be 12-14 volts DC there.
Then move the red probe to the bottom of the lower red wire terminal and there should be 12-14 volts there too.
If you have 12-14 volts on the bottom wire terminal, the water heater control board is your next stop. Troubleshooting past the thermostat is out of the "average handyman" repair since replacing the control board could get complicated.
My best guess you will find your problem in the thermostat or power center fuse. BOTH are relatively easy to change.
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