as duct work leaves furnace it branches 2 directions, only one direction is getting warm air. The quot;tquot; in t he duct work is directly above furnace. There is like a 20 degree difference in temp of air coming out of registers. Plenty of air flow to the cold side just no warmth.
If it is a gas furnace I would guess that some of your burners are not lit. Or something is plugging the airflow inside the combustion chamber, or that side of the heat exchanger. Might be a bat came down, died, and is blocking air flow. Get a professional to look at it ASAP
