If you have verified the water heater is plugged into a power source and it does indeed have power......the you need to see if there is power at the electric heating element end.
If not, then another common problem is the hi-temp thermostat get's burned out. This is the resettable button underneath the rubber cover on the outside of the water heater that you said you have pushed the button on in your OP.
You should have two of the resettable buttons, if you have the Suburban brand water heater. One is for the 120 volt side, and the other is for the 12 volt side.
These hi-temp thermostats act like a fuse, and if they burn out, it will keep you from getting electricity to the element. It's also very common for these to burn out if the electric heating element burns out. You may replace the electric heating element, but it won't work until the hi-limit thermostat is also replaced.
Herk has a great and detailed explanation of all this which can be found here:
Suburban Water Heater Reset Button
Here are pics of the thermostats, and with the rubber cover removed...that shows the burned out 120 volt side and the good 12 volt side.