The water thermostat has been turned right down on the front of the boiler?
Or maybe you have a switch on the boiler that enables you to turn the heating only on (mine only has a switch to do it the other way round, but worth checking)
Quite often it's the diverter valve. As the name suggests, it just shoots the hot water in one direction for heating rads, or the other way to provide hot water.
Needs looking at professionally though :o)