There is a relay operated by 12V
that switches the 120v heating element on. Listen when you turn on the switch to turn the heater on in 120v mode. You should hear a click as the relay closes and allows current to flow through the electric element The relay is on the back of the water heater close to the bottom under a black cover. The yellow wire is the 12V
wire that carries current to the relay. If you are hearing the relay click, carefully check that you have 120V coming in to the water heater (your breaker is on-right) If there is current there (put probes of volt meter on black and white wires and it should show 120V AC) check the element for continuity, it may be burned out if you accidentally turned on the power with a dry tank. Still an easy fix if you have access to the back of the water heater.