My water heater keeps cutting off.the breaker switch doesn't flip so. Can anyone help solve my problem?

Measure voltage into unit, through the red reset button, then through thermostat, thermostat is probably problem if adjustment won't heat to temperature, If dual Element, one may be burned out. I like volt meter, and amprobe for testing. If Element has voltage,
And does not draw current, it is open. No need to disconnect element to test.

Measure voltage into unit, through the red reset button, then through thermostat, thermostat is probably problem if adjustment won't heat to temperature, If dual Element, one may be burned out. I like volt meter, and amprobe for testing. If Element has voltage,
and does not draw current, it is open. No need to disconnect element to test.
I just tried it and there's voltage.

Is there current?
Did you try thermostat, while reading voltage and see if it kicks in and out, as when you turn temp up or down?
Do you have 2 Elements?
May be bad thermostat.