If you have no hot waterStart by checking the switch in the hot water cupboard has not been switched off. The next option is to check the circuit breaker on your switchboard has not tripped. This is normally labelled hot water. If both of these options are fine, then a) the thermostat or element (or both) has failed and will need to be replaced. Or b) the ripple control has tripped. The ripple control is a circuit normally on the main circuit board. It can be switched on and off by the power board to manage peak energy flows. These can fail sometimes, and your utility power supplier will come and replace it. An electrician can also get you up and running.