When you mount a non-AI lens on a camera like the D800, the metering tab on the camera and the aperture ring on the lens try to occupy the same space. Needless to say, something has to give.
Sometimes the metering tab gets damaged and the camera will need repair to work with AI lenses. More often, though, tolerances and wear allow the aperture ring to "ride over" the metering tab. That may have been the resistance you were feeling when you first tried to mount the lens, and may account for the incorrect exposure. (Other things, such as setting the wrong maximum aperture, could also contribute to the underexposure.)
I suggest visually inspecting your camera and trying an AI lens, but if you were able to remove the lens cleanly there's a good chance you didn't do any permanent damage.
"There is no real magic in photography, just the sloppy intersection of physics and art." — Kirk Tuck