does the system have a motorised valve fitted ?? this sounds like an electrical fault, maybe a faulty motorised valve.

I would say your motorized valve has failed...located in the airing cupboard, you may have two dependant on the system fitted...plumber required really.

I agree with the answers provided by ABS & AMB that it is likely to be a faulty motorized valve. It will generally be a galvanized metal box mounted on the pipework near the tank.
There will probably be an overide lever on the side.

If it has failed electrically it is possible to change just the motor, that will save the need of a plumber and breaking into the system, plus it is cheaper! If the valve itself has siezed it will need the whole valve replacing.

agree with above, more often its build up of sludge in system that jams the valve, usually need new valve and flush system

will be a midport / 3 way valve (if it always worked previously), that isnt moving over to the heating position. you have only shared and hot water positions on the valve. if it never worked independently then you may have a wiring fault