I'm guessing this is in a car, truck, or van since it is in that category.
- Diverter door malfunction: Most if not all cars use a diverter door to push air past either the Heater Core, or Evap (air conditioning); these doors are controlled by a vacuum pot (hose and line) in most cars, and by electric means (In rare instances). You will need to find specifics about 'your car' to tell what type, as well as its location to see if 'that' is the problem.
- Some cars use a valve on the heater line (the line that allows hot water to enter the heater core) if this valve is damaged, or stops working, then hot water will not reach the core to provide heat.
- There is always the case that someone has bypassed the heater core (disconnected it) as well.
That should give you enough information to start at least.
