The image captured on photographic film is called a latent image. Translated, this means an image invisible unless somehow treated to cause a visible image to appear. The latent image is best when the treatment (developing process) occurs soon after exposure. As time passes the latent image weakens. This decline is due to chemical changes and outside factors. Film is not just sensitive to light, it is effected by heat and naturally occurring radiations.
No doubt, if images were captured by this film, they are lost. Let’s explore a little further. All film, as it is manufactured has edge printing applied. These are numbers, symbols, and letters that are imprinted along the edge of the film. Edge printing is applied with a “light printer”. These symbols are exposed using light and their image is also latent.
When the film is developed, the latent images made by the camera are developed and made visible. As these photographic images appear, so does the edge printing. I tell you this because you should examine the negatives and see for yourself if edges show edge printing. If the edge printing is bold, you can rest assured that the film was processes OK and that the lack of photographic images is because no suitable exposures were made on the film. If the edge printing is super weak or not present, now you can suspect that film was damaged due to its age or the photofinisher misprocessed.
Sorry this project did not yield good results. It is unlikely that anyone can salvage images from this processed film.