Visions of a hot bath or shower after a hard day's work …

… vanishes as the water flowing, with the mixer set to the usual hot water side (or form the usual hot water tap), remains cold.
Unfortunately hot-water geyser-elements and thermostats only have a certain lifespan, which get drastically shortened if the water-supply in the area is interrupted for some or other reason and the elements of the geyser get exposed due to water usage/drainage. The geyser's thermostat regulates the element, by switching it on when the water-temperature drops below a certain level and switches off when the water reaches a certain temperature. As water is drained from the geyser, the geyser fills up and both the element and thermostat remains emerged in water. If for some reason the water do not fill up, it is best to switch the geyser off to protect the element, until the geyser has filled up again.
It is not necessary to replace the geyser if either the element of thermostat (or both) no longer works.
Switch the geyser off first.
Thereafter empty the geyser, by linking a pipe to the valve and then opening the valve to let the water run out to an appropriate place, for example the garden.
Undo the wiring to the element and mark which side goes where.
A special geyser spanner and substantial leverage is usually necessary to loosen the element.
Remove the element and buy a replacement element of the correct kind.

Replace the element and tighten it with the special geyser spanner.
Fill the geyser, while reconnecting the wiring and once filled, switch it back on. If the thermostat is still working, the geyser should now produce hot water again.
Replacing the thermostat is less of an hassle.

The geyser need not be emptied, but first switch the geyser off. Uncouple the wiring and pull the thermostat out of its tube-socket.
Push the new thermostat in, reconnect the wiring and switch it back on.