If the heater water system is clear and the pump isn't man enough to push water through, then the pump could be the fault, but that seems unlikely unless the impellor is loose inside it. In that case the engine would overheat. Is the temperature gauge getting up to normal? If not, it could be the thermostat stuck open? That would keep the water cool and minimise flow to the matrix.
Far more likely is a blockage. Take the hose off where the water goes in to the heater valve and on the return and flush that with a hose....