If one lens makes the viewfinder very dark but another lens works fine, the problem is likely in that lens rather than the camera body. The most likely cause is that the aperture is stuck stopped down to a very small opening. On Nikon G lenses, this can happen if the aperture lever is stuck, bent, broken, or out of alignment, so the lens stays at minimum aperture instead of opening normally when mounted. Before sending it for repair, try cleaning the lens electrical contacts in case the camera is getting incorrect aperture information. If your lens has an aperture ring, also check that it hasn’t been moved accidentally. If the lens still stays dark after those checks, it likely needs service or replacement.