Simple all you need is a toolkit that contains , continuity tester, tube spanners , cutting plier , tester and screw driver it's simple once you have the above tools 1 St take your continuity tester and check the phase and neutral nodes on you let water heater plug if you see a light it seems your water heater is working fine now test the phase node along with the earth node if you still see the light the heating element inside your water seems damaged and needs to be replaced if you don't see the light you have a thermostat issue which can be replaced and this should sort your problem once replaced.