Tepid water temperature

The things to check first are.

1 That the main valve coming into your house is fully open.

2 Then check that the valve on the line to the shower is fully open.

If these valves are not fully open then there may not be enough water
getting to your shower to open / close the safety device that allows power through to the elements

Make sure that the shower head is clean and clear, that the hose isn’t kinked (this can happen just behind the hand set where it is difficult to see) also check the inside of the hose although it can look OK on the outside it could have collapsed internally

The other possibility is a faulty flow/stabilizer valve not allowing the correct amount of water into the shower.

A ruff check is to remove the shower head turn temperature control to fully hot the water coming out of the hose should be a bit more than a dribble then turn to fully cold and it should be coming out at full flow.

If there is not much difference between the two then you could have a faulty flow valve.

Carry out this check after you check the inlet valves.

Also if it is a Triton have a look at the outlet TCO

The outlet Thermal Cut Out this is a black disk with two wires going to it and it is mounted on the out let pipe (The bit between the heating tank and the hose)
Examine the connections closely if you see any discoloration / signs of overheating then the TCO needs replaced.

The other possibility is a faulty element in the heating tank (there are two, three in most 10kw showers) you can check them with a multi meter you should have a reading of around 12>18 ohms.

If you have power to the elements and no heat it's probably the elements, no power to the elements then it's probably a micro switch.

If you don’t have a multimeter  we sell them at £10 each with a guide on how to use them for checking and testing the various parts of your shower.

Apart from helping to fix your shower, they are really useful for checking you have a power supply to appliances, for checking fuses, letting you know much power you have left in batteries

(so you know if they are dud or not)  and lots of other jobs, in short they are well  worth the £10.

http://www.showerdoc.com/installation/electrical-products/SKYTRO-600-005-Digital-Multi-Tester