Heating but no hot water?

I have a glowworm 30 Ci combi boiler which has been great since it was installed.
but yesterday the hot water stopped running hot and is permanently cold.
The boiler pressure has stayed at 1.3 bar and the radiators heat up if i turn the heating on, its just when any hot tap is turned on the boiler does not fire up.
My girlfriend said that she had a shower this morning and it was hot all the time (about 15 mins) but when i tried (about three hours later) the basin taps or the bath or the shower i could get no hot water at all.

There has been no hot water at all no matter what we do, at any time of day or night

The pressure is still ok and the heating comes on fine, just no hot water.

anybody had similar, or know what prob might be???

any help greatly appreciated.

thanks.