This morning I had a shower.. no problem the water was piping hot as usual I got ready and went to work. My wife, who works nights, got up a few hours later and also had a shower... no problem. When my wife tried to run the hot water tap a few hours later to do some washing up there was no water coming from the tap at all. It has been cold... but water was coming out of the hot water tap as normal when I got up at 7am. There has benn no unusual demand on the system we have no guests or excessive demands. Can you suggest a possible cause??