Achieving zero airflow from a specific supply register in a room is typically the result of an entirely disconnected duct run, a completely closed balancing damper, a crushed and blocked flexible duct, or a blocked boot. If an animal or construction debris has fallen into the ductwork, or if a flexible tube has sheared off its takeoff collar in the attic, the conditioned air will spill into unconditioned structural cavities instead of exiting the register. You can troubleshoot by removing the room's register grille and feeling inside the boot for obstructions or checking the main trunk line in your basement or crawlspace. You can verify that you have successfully restored zero-airflow rooms to normal operation when repairing the severed duct connection or opening closed dampers allows strong, chilled air to blow freely from the register.
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