Your heater will blow cold air only.
you may not have the heater water hoses plugged in
replace front heater corefront and rear have independent heater cores
Sounds like the Blend Door actuator, a $6.00 part but about $2,000 in labor, the dash has to be removed to do the job right.
Thermostat and/or heater core may be defective
check your coolant level
Low coolant in the system. Defective heater control valve. Clogged heater core.
You are low on coolant to the heater core.
check thermostat, might be stuck closed.
Check for low coolant
Possible causes are, low on coolant, thermostat stuck open, clogged heater core or vacuum scorce not getting to the heater control.