My 2000 Audi A4 front passenger door won't door won't open.

The door might not be opening because the window fell onto the bar that actuates the door lock and broke it or pulled it out of its' armm. You can probably get to it by rmoving some trim panels as you would to get to the windows anyway.
Either the trim panels are held on by screws, sometimes things like the door handle might have a plastic dip or cup to grab onto to pull, it may be held on by screws which retain the trim panel. The same goes for all the switches as they must be removed. Once all screw secured items are removed, the rest is simply pulled out as it is held in place with plugs that are re-usable most of the time. You can get a tool to aid in removing the trim panels.
Once removed you may have to peel back some plastic which acts as a sound deadining device, you will never notice the difference, and then you will begin to see the iner working of the window, the rails it is guided by and the bar that is its backbone to hold it up. That is usually what comes loose. Look for the screw in the bottom of the door with a magnetic pick up and it generally screws into the inner door frame and into the window backing frame. Then look for the bar that runs from the door handles to the door mechanism. You will probably find that they are pulled out of socket and need to be put back in.