A dark band at the top of every photo on a DSLR like the Nikon D5100 usually points to a shutter problem, most likely a sticky or misaligned shutter that is not opening or closing fully.

Because the optical viewfinder uses the mirror path rather than the sensor path, a normal-looking viewfinder with a dark band in the recorded image strongly suggests the issue is happening at the shutter, not in the viewfinder system.

In most cases, this means the camera needs service. If the camera was recently dropped or knocked, the shutter may have been jarred out of alignment.

You may be able to inspect the shutter area with the lens removed, but attempting to free or adjust it yourself is risky and can make the damage worse. Unless you are prepared to accept that risk, professional repair is the safer option.