Whenever we lose heating or water or both together, its normally down to the electrical side of the installation - specifically the motorised valves.
In our system, one boiler heats both hot water and the radiators.
To make them independant (so that the heating comes on when we need it and the water comes on when we need the water heated and not necessarily at the same time) the controller (the beige box that allows us to specify what time things come on and off - ours is a Honeywell I believe) controls two electrical valves.
These are plumbed into the system pipework but the electrical motors that operate the valves are external.
They are easy to replace for anyone confident with electrical work (someone who would know how to install a plug socket for example).
It could be that one of these motors is shot - maybe it happened at the same time that the emersion went pop.
Trace the wiring from the control box, this will go to two tin/aliminium coloured boxes about 2 inches square on different legs of the pipework - mine is near to the system pump (a large red block with an arrow on the front indicating direction of the pump action).
You should be able to detect a noise as the working one shuts off and on if your listen carefully.
I am not a plumber so this is not professional advice !
I am lucky that all my pipework is easy to get to.
In my (limited) experience, the motorised valves can be opened manually. If you open the valve and your water starts to heat up, then that will help with the final answer
I had the same problem, and it was the motor on my valve that had gone. I bought a new one from B&Q (about £18, 3 years ago) and fitted it myself. Not too bad a job.
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