Audi A4 car door window us not working

can you hear the motor trying to work the window? A fairly common problem is the window regulator breaks. this is the cable operated lifting mechanism that raises the window, when it breaks you can hear the motor winding away, accompanied by a faint clunking noise as the motor tries to operate the mechanism. They are quite difficult to replace.
if you cant hear the motor working (also assuming all your other windows do work) then either to motor is faulty or there is no power to the motor. the wiring inside the rubber boot that goes from the car to the door (by the door hinges) is prone to breaking due to the constant flexing back and fourth as the door is opened and closed. switches can also fail, you may be able to swap the drivers/passengers switches around if they are identical for testing.