I need advice please. Britony Combi 80 boiler no hot water only C/H now?

There is no hot water coming from our Britony Combi 80. When I turn on the hot water tap the water remains cold and all the hot water is going to the radiators. This happens even with the front control knob is on 'Hot Water' only, and I can no longer hear the small rumbling-like noise when the flow method was changing over. I would be please to receive any advice no matter how small. Many thanks