thermostst is stuck
Low coolant, bad coolant, blockage in the coolant line, bad heater core or element, bad thermostat
If it is coolant, a leaky heater core. If it is water, a clogged ac drain or damaged door gasket.
Low coolant. If the coolant is low then the heater core is not able to transfer heat in to the car.
You have a coolant leak somewhere. Fill and check underneath for any obvious leakage.
Never flushing & replacing your anti-freeze/coolant...
Broken or split hose Radiator leak heater core leak
Maybe the heater core is leaking.
Low coolant.
Heater core may be air bound and have to be bled Heater core may be plugged or restricted Thermostat may not be opening allowing coolant flow to core
If your heater core is LEAKING engine coolant so that your engine coolant level drops too low or the engine coolant is lost completely and causes your engine to overheat then your engine can be damaged beside the mess the leaking coolant makes inside your vehicle ( you might be able to just temporarily bypass the heater core so that no engine coolant is flowing through the heater core )
When a head gasket blows, it most often causes a lose of coolant either in the exhaust or the oil. When the coolant drops, there is not enough to circulate through the heater core. The engine may over heat but no heat will come out of the heater because there is no coolant in it.
Not enough coolant is running through the system chek for leaks and chek your heater core