WHY HAS MY HEATER STOPPED WORKING?
The most common cause of a heater failing to work is one of a lack of water flow.
This is caused by the filter or pump strainer basket being too dirty to allow enough water flow to activate the heater pressure switch which in turn will not activate the heater.
Remedy: see Backwashing & Cleaning of the pump course strainer.
If the above does not work you will need a service call.