Water hot in pipes but nothing out of taps?!?

Hi there, the hot water just stopped when I was in the shower and now nothing comes out of any how water taps or the shower. The cold circuit seems unaffected. The circulating pump seems to be whirring away and is very hot as is the water near the boiler, which fires up fine. There is a box of water above the boiler but only a very small amount trickling into it occasionally. The pipes by the cylinder in the next cupboard are not hot at all. Apparently the circulating pump is pretty new. Is there some air blocked? Or some valve or switch not functioning correctly? I'm normally pretty handy at these things but cannot fathom the endless pipes to isolate the problem - any help would be very very greatfully received!! Thanks, Paul