When on autofocus my camera will not focus. The lens will do the twisting movement, go in and out of focus and then stay out of focus. Then it won't even take a photo. In addition, when on manual focus, it will take a photo, though the photo is pitch black. Mirrors look fine. When I use the lever for the screen to display the photo that I'm about to take, it is also pitch black, though I can see a thin line on the bottom where the photo is coming through.
It seems like your mirror may be locking up. See this brief tutorial pointing to the issue, or this one using the D610 (the actual repair is shown at ~3:10). Note that any attempt to repair that could well destroy the mirror mechanism, but, considering the cost of professional repair, it might be worth a try if you consider the camera already useless.