No recalls on this, FYI.
Roy has a good thought on the heater core, but you could also have a thermostat issue causing the engine not to get to full operating temperature.
You mention the air "turned cold" - which is much different than saying it is "not hot". If it's truly cold, you also may want to investigate whether or not the temperature control is at fault. Is this car equipped with the optional Electronic Automatic Temperature Control (EATC)? If so, temperature sensors and/or a temperature blend door motor could be at fault.
For the heater core, with the car at operating temperature, carefully check how hot the inlet and outlet hoses to the heater core actually are. If one is hot and the other is not, then the restricted heater core issue is a very likely reality.