Is it blowing air?
Do you just get cold air blown through? or is no air coming through at all?
If the matrix is hot....and the fan is working....can't see why you wouldn't get heat through.
If there is no air coming through, then it is the fan or a baffle.
If air is coming through, then I would really make sure the matrix has water in it and is hot.
Worry is....finding the air running cold through the heater can often be caused by a loss of coolant, which may be caused by a burst head gasket....An overheating engine with cold air from heater is often the first sign of head gasket failure.
eib