Hot then cool.

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 allowspower through to the elements

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

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

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

Apart from helping to fix yourshower, 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 ornot)  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