No electric current going into Water Heater
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.