Right at the end of the water tank there should be a small electrical box to which there should be a small red reset button. The tank needs to be full of water first as it has a built in safety feature which stops the electric supply activating and burning out the element. Once the tank is full of water and the electric supply connected, you should hear a click when the reset button is pressed. If it doesn't then check at the RCD to see if the trip has switched off.
If you have a duff heater element all is not lost with a bit of work.
Carver Cascade immersion heater repair (December 2015)
It is possible to reclaim a faulty immersion heater, first check to see if their is continuity on the element should be about 80Ω then check for insulation with a 500v Megger you will have to dry out the immersion heater to get a good test reading. When it tests OK clean of the crud with a dremel with wire brush, give the damaged area a thin layer of J B Water Weld and finish with over with high temp enamel paint (car engine/brake paint) p.s this water is not used for drinking. I have just restored mine to working condition and tested over two days heating and cooling, not yet installed in van but everything looks good for next year.
P.S. this has now been in use for two weeks with no further problems (April 2016).