Three elements to this. One could be that there is sludge in the bottom of the heater matrix under the dash blocking the flow from the engine; another is that the valve between the engine coolant pipes and the heater matrix has failed; and the other is that the fan motor itself has failed. Do not attempt any bodgery because this is what caused Zafira Bs to catch fire. Replacing a heater matrix is a very slow, frustrating job because it can mean that the whole dash has to come out to gain access.