Are you talking about the middle vents? As they are not heated, they will only blow cold air. Only the vents below the windscreen and in the footwell provide heated air.
If these also remain cold, check the hot water supply behind your ventilation unit. Two black hoses with a white plastic valve. If this doesnt move, this might be your problem, but you can open it by hand.
If the hoses are hot, but there is still no hot water, you might have air bubbles in your cooling system which blocks the fluid from entering the heater radiator.
These are the standard things i know, but there might be other causes... Good luck!