When my car failed to deliver hot air to the cabin the cause was a broken demist flap that had fallen into the heater and jammed the upper blend door in the “cold” position. This prevented the air being diverted through the heater matrix so only delivered cold air through the side and screen vents.
A small amount of heat did appear at the passenger side footwell, but none at drivers side due to a failing blower also being an issue at the time.