Do not force the lens. If a Nikon F-mount lens suddenly stops seating or rotating into the locked position, inspect the lens mount carefully for anything protruding or out of place. In this case, the cause was a slightly loose screw on the lens mount ring, which prevented the lens from seating correctly. Check both the lens and camera mount for loose screws, damage, dirt, or anything obstructing the mount. If you find a loose screw, the safest fix is to have it tightened properly rather than forcing the lens onto the body, which could damage the camera or lens mount.