more than likely the diverter valve, has said before its either a little silver metal or a white platic box with an electric cable coming out of it and its attached to the pipework.
this valve rests in the hot water position and an electric motor moves it over for heating.
there should be a small metal lever which can be sed to manually move the valve over, move this lever into the heating position and see if the boiler fires up (make sure the water is off on the programer)
if it does the motor in the valve or the valve itself needs replacing.