Probably going to have to get it from Nikon or send it to them for repair. But for a D200 probably cost more to repair than to but a used D200. a used one at Adorama sells for $144 and B&H has one for $199.
Or even better take this opportunity to upgrade to a new camera
You probably just opened the spring that holds the focusing screen in place. When I replaced my focusing screens, the replacement came with a special tool to unhook the spring that holds the screen. You probably just would have to push the spring back into place to hold the screen. Nothing seems to be missing from the picture you supplied.
You could go to link and look at their instructions to replace a focusing screen. It might show you how to fix your camera. Hope it works for you.
I think that in all Nikon DSLR, you can replace the focusing screen. Yours must have fallen out due to weak springs which just released the screen. Search the NET to figure out how to put the screen back. KatzEye and other third party focusing screen suppliers have step by step graphics for this purpose.
It may be good to take this opportunity to install a third-party focus screen. I don't know what the stock screen is like in a D200, but the one in a D3200 is pretty much useless for actually focusing; and if the screen in a D200 is similar, then for a bit over $20, an aftermarket focus screen would be a rather significant upgrade.