Possible Causes

WARNING: This page is about a different car, the 2002 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.

Low engine coolant level.
Engine overheating.
Heater control valve.
Plugged or partially plugged heater core.
Temperature blend door binding/stuck.
Temperature blend door actuator.
Temperature blend door control cable damaged/disconnected.