Hard saying without looking at it but things it could be are a plugged heater core, a bad thermostat, the HVAC Blend Door Actuator, or even the Panel Mode Door Actuator. Should take it to a trusted shop for repair.
If you are getting the defrost to work and hot air is coming out, then it could be a bad blend door actuator, or bad Panel Mode Door Actuator. if everything is cold then plugged heater core, you can try flush it with this stuff/ http://www.reactionchemicals.com/pdf/restricted-clogged-heater- core-procedure.pdf
If you antifreeze is not up to snuff then frozen coolant also could be the cause. Frozen coolant will cause overheating as well. Not sure what part of the country you are in. Judging by the local trees, am guessing you are in the Carolinas perhaps?
If you are on the East coast then it is probably not cold enough for the coolant to be frozen unless it is all water.
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