My answer assumed yours is a conventional basement installation where you have a duct running under the ceiling from the furnace and ducts to each room branching off it. Then there is a second duct collecting the returns and returning the air to the furnace. Regardless of where the ducts are, the one that is not connected needs to be connected allowing air to return the furnace.

Many furnaces are set up to adjust the speed of the fan to give the correct delta temperature depending on the air flow. Yours should be checked and adjusted if needed after fixing the duct.