Possibly, the 3-way valve that diverts hot water to the taps etc instead of the rads, has stuck. It could be a circuit board problem, or even a faulty HW thermostat.
Loads of possibilities. Boiler engineer needed (may be out of the scope of a regular plumber.)
ok thanks guys!! Better look for a plumber! (heating engineer) we,ve just found that if we turn the radiator temperature down to min that eventually hot water will come out of the taps!!! THANKS AGAIN !!!