1. Low coolant level
2. Air in cooling system (did you drain it recently?)
3. non-functioning heater valve doesn't allow hot coolant to enter heater core.
4. non-opening blender door to direct heat to the passenger compartment
If you can feel the heater hoses, one IN and one OUT, and they are both warm, then it's probably the blending door. If one HOT one COLD, probably the heater valve. If neither one HOT, then probably low coolant or air in system.