Most likely you need to sip in more coolant, the coolant liquid level is probably too low to reach the heating radiator.
If you do have enough coolant, then you need to bleed the coolant, there is probably an air bubble or something clogging the pipes. As the last resort, you should disconect the heating radiator coolant inlet and outlet, and let a bit pressurised water through the OUTLET (the oposit of the usual flow).
But my guess is you simply have insufficient coolant level.