My Worcester 24i Combi was working perfecly until I ran hot water tap?

Last night my Worcester 24i Combi was working O.K. until I turned on a hot water domestic tap. However, after briefly getting a small amount of hot water from the tap, it then ran cold. The central heating rads have all now gone cold and no hot water is available. After looking at the Internet, I checked the water pressure and adjusted it to 1.5. There is gas to the appliance, electric feed to the boiler (light is on), clean burners & the thermocouples appear to be in good order. The pump sounds as though it is coming on whenever I open the hot water taps or try to put the central heating on, but neither function appears to be warming up. Any ideas please?