Hello , I have a furnace in my basement of the 5 vents only one blows cold air? I can see the air ducts and there is no leak.

Very strange. If the duct that is blowing cold is connected to the main duct that is blowing hot I cannot see how it would not be working unless it is a return air duct and they can feel like they are blowing cold but are actually taking cold ait back to the furnace. You might also see if there is a damper on the pipe that is closed.