1) Engine is out of water, which means will be dead soon.
2) More often thermostat is stuck open, so engine never warms up properly
3) The water valve which directs water into the interior heater core is stuck shut
4) You have AC running AND outside cold air on VENT setting, but even that should allow some warmth.
5) You have the temp control turned to COLD.
Why does my Pontiac Grand am blow out cold air