Perform a hard reset to set a good reference point. Please see the attached photo with instructions for the procedure.
If no improvement, remove the lens, memory card and battery, set the mode dial to M (manual), and switch the camera to the ON position. Allow the camera to sit idle for 20 minutes. Then, switch the camera to OFF, re install the battery, memory card and lens, switch it on and try to fire the shutter while still in M (manual) mode. If the viewfinder is still black, remove the lens, put the camera in manual and while observing the mirror press the shutter button and see if there is any movement at all. Please let me know the result
Please keep in mind that my diagnosis & solutions provided are directly dependent on the accuracy of your description of the problem. As with any "do it yourself" fixes, success is a "team effort", since I can't see or touch the camera, and relies on the customer's manual dexterity and ability to follow the instructions well. If the "do it yourself" approach doesn't work out for you and you need conventional repair, please let me know.
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Do you have access to another lens to try? So far, the symptoms point to the lens aperture not functioning correctly?
If you take a shot, with the mode dial set to manual and the lens removed, does the camera still produce a black frame or is it all white?
Unfortunately, if only black is produced with the full amount of light provided in this test, the sensor has failed. This is not something that should happen with a camera of this quality unless it has suffered an impact or was exposed to excessive heat or cold. The only reasonable option is Nikon repair. You can contact them at the link below.
Every symptom indicates that the, image sensor housed in the body, has failed, as indicated in my last message to you. There is no simple user fix for this issue, and, I am sorry, the sensor must be replaced.