The symptom indicates a sub-mirror failure. This can happen at any number of actuations, but, generally somewhere between 15k & 100k.
Unfortunately there are no simple user fixes for this issue and this problem will require professional service.
The service below is a good third party service. The repair falls under Shutter service.
You can contact Nikon at the following link
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.
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