If your negative film is totally black or your slide film is totally clear, it has been completely overexposed to light. If the overexposure is limited to the film frame area, and does not cover the film edge marks, the problem is probably operator error or camera malfunction. If the film is exposed edge to edge, covering all the edge markings, the film was probably exposed to light outside of the camera, or by opening the camera during or before the film was rewound into the film cassette.