[puddlesplasher]

2 things spring to mind:

1) As discussed above, try topping up the radiator reservoir. My Son ran his car cold a few Winters past and was relieved to learn it was only a top-up that was required. A quick fill with collant and the car was blowing hot air again.

2) Not for the faint hearted, "door blender switches", located behind the dash console are for an easy description, little air flow flaps. When opened you get the flow of hot air and closed gives cold air. Your switch could be jammed in the partial open position. thereby allowing the almost constant temperature of air.

Both can be fixed but I would try the top-up first.