As you can see the viewfinder information that tells us the finder shutter is not closed (broken).
This sounds like your reflex mirror linkage broke so the mirror is not going back down to the 45º angle. And with the rattling there could be either a screw floating around or the link arm has snapped.
You’ll need to grab the Nikon D800 repair manual if you are going to try to fix this on your own. I’ll warn you this is a very delicate repair as you will need to take the camera almost completely apart. Unless you have the skills to work on small fragile parts and have micro-soldering skills I would recommend you find someone with the needed deeper skills a local camera repair person or shipping it off to Nikon for repair.