Worcester GreenStarHE Combi Boiler: No Hot Water from Taps?

We have a Worcester GreenStarHE Combi Boiler (4 years old). Today, out of the blue, hot water no longer comes out of the taps. The boiler is providing central heating, when the water demand is there, the red pilot led comes on, but no hot water comes out of the taps. We have tried shutting the boiler down and restarting, but to no avail.