Hanging Man

A man is seen hanging by the neck from a single beam. The room is completely empty. The only door is locked and bolted with the key still in the lock on the inside of the door. The only window is also closed from inside. There are no signs of a forced entry. Time of death is estimated to be appr. 8 hours ago. There was a damp patch on the floor which later proved to be water — not blood.
How did the man die?