Problem with print

It sounds like your shutter may be failing.

The N80 has a vertical travel focal plane shutter. This is essentially two curtains that travel across the film plane. At lower shutter speeds, one curtain opens up the shurter, you wait, then the next one travels across, stopping the exposure. At faster speeds, though, they both move together, leaving a small gap between them to let light through.

If the first shutter curtain isn't always making it all the way to the top, this could give you just what you're seeing -- a black band across the top of the photo. One blank negative may well indicate that your shutter failed completely on that shot.

This is something best handled by a camera repair shop. It could possibly just be some dirt in the shutter mechanism, which would be fixed by a good CLA (clean, lube, adjust) ... that's no a bad idea if it's an older camera anyway. However, it could be more than that. It won't be clear which without getting down into the camera itself.