If you have the GeyserWise system, that uses a 12 volt pump that has stopped working, it is most likely the 12 volt battery that drives the pump; that has failed.
Unfortunately, the GeyserWise display does not have a battery indicator, to tell if you if the battery has a problem.
The 12 volt battery voltage is topped up by a small solar panel during the day. It is mounted next to the tube collectors on the roof.
At night when there is no sun, then your Geyser will be using the electric power from Eskom.
During the day however, when the sun is out and about and the collector temperature heats up, then the GeyserWise system will start the pump when the temperature difference between the roof collectors and Geyser is more than 7°C (default setting).

When this happens, the GeyserWise control box turns on the pump which then draws the power from the 12 volt battery.
If you have to change the battery, I suggest that you use a Lithium battery.