A sharp black area on just one frame usually means something blocked light from reaching part of the film very close to the film plane. Based on the answers, the most likely causes are:

• Shutter malfunction: If the shutter didn’t fully travel for that exposure, part of the frame would stay unexposed, often with a hard edge.
• Film advance issue: If the film was not advanced correctly, only part of the frame may have been positioned for exposure.
• Camera back briefly opened: Less likely, but partial exposure during handling can affect part of a frame depending on how the film was sitting.
• Object very close to the lens: Possible, but less likely if the edge is extremely sharp.

Since only one frame is affected and the area is black rather than washed out, a one-off shutter or film-transport issue is the best fit. A good check is to remove the lens, open the camera with no film loaded, and fire the shutter while watching whether it moves smoothly and fully across the frame.