We bought a brand new water heater back in November. Came home last night to no hot water. Check circuit breaker and it wasn't tripped. Turned it off an on anyway. Nothing. Hit reset button on thermostat, nothing. Check for electric current per manufacturers instructions and there was nothing. Put in a new circuit breaker. Still nothing. What else could it be?
Since this is brand new, and assuming you hit the red reset button on the UPPER thermostat, then it's most likely a loose wire somewhere. It might have been touching a terminal and worked for a while, but under load that loose connection over heated and burned away. One bad connection and the entire circuit is broken, so unless you know how to test for that, you can easily get a false negative reading. Here is what your entire circuit looks like.
If no power at the junction box and you installed a new breaker did you verify there is power on the breaker? Most water heaters are double pole you should see 240v at the 2 wires coming out of the breaker. If there is no power there it can be a bad buss. I have seen this with 2 brands FPE stab lock panels made by federal Pacific and zinsco panels. If there is not 240 at the breaker check the main breaker, if you lost a leg I would expect you to notice other circuits not working but worth a check.