I found that my furnace is leaking a lot of water from the vent connected to the furnace unit .the pump next to the furnace is also empty .please help on what causing that ?

Are you calling the condensate drain a "vent "pipe, the plastic tube that runs to the small pump? If so, the pipe is blocked with debris or the condensate tray drain hole is blocked. You may be able to fish a wire back through the drain or open the plenum on the air handler and clear the blockage.