Why do you have a hot water heater? Surely, there's no point in heating the water if it's already hot.
Seriously though, if you've already replaced the heater elements, then you need to check the thermostats (if they are separate to the elements).
There may be several thermostats - e.g. one for normal temperature control which switches the heater on and off as required, and a backup one that shuts off and stays off until manually reset, if the temperature goes above a safe level.
Next check the wiring back to the wall, and the outlet where the power is taken from. Test to see if there is power at the outlet using a test lamp or multimeter.
If there is no power there, you need to trace the wiring back to the panel, and ensure that the breakers are switched on, working, and that the wiring is intact.