Heating works, but Hot Water doesn't... Glowworm Micron 40FF?

I've having a strange problem at home.

If, via the Programmer, I turn the heating on the radiators come on.
If, via the Programmer, I turn the hot water on, I get nothing, nada, zilch.

Its only just happened, the water is clearly heating up as the radiators are working, but there is nothing coming out of the tap.

Any ideas what this could be? I wondered about the diapgram or 3 port motorised valve, I wouldn't have a clue how to fix either of these. Any ideas how much it'll set me back?

Glow-Worm Micron 40FF