Check the easy things first;
make sure the heater "tap" opens and closes, seen the cable/wire broken off the taps a few times on older cars.
make sure that there are no air locks in the heater, often happens when coolant level has been allowed to drop, and been topped up with the heater off.
check that the thermostat is closing - engine temp staying cold after driving is a good sign that there are problems and that the coolant cant get up to operating temp.
fuse blown on the fan, if relying on blown air to get the heat out.
After that start worrying about the heater matrix.