904 automatic transmission not shifting on any gears

I assume you have checked the easy thing like shift linkage hasn't came undone? If it is still connected and you still have the pan off have someone cycle the shift linkage while your under there looking at the manual valve and check that it's moving in and out of the valve body like it should. If it is it's possible that a piece of crap found it's way into the main pressure regulator valve holding it open. It's located in the valve body.