Changed lenses on my Nikon D2X and the vie...

Take the lens off and look at the front of the camera body...on the left side ( 9 0'clock position ) is the stop down lever. Check if it looks bent to the side. It's pretty flimsy and can get bent out of shape. The top of that lever needs to be horizontal to catch the lens post.

If the lever is lower than the 9 0clock position, the substrate unit needs to be replaced.