Heater woes

I've only had my GL450 for a couple of months. I have set the climate control to auto, yet this morning I didn't seem to get any heat in the floor vents. If I had the rear vents in the console on, I could feel heat, but the front floor well blew cold air only which was not comfortable with the 16 degree temps. Any ideas on what I should look to replace/repair? Is this a possible blend door issue? Any thoughts will be appreciated.