hiya, im not entirely certain but i work for a digital wedding photographer and we have similar problems with a couple of images and i think it usually means that they are corrupted files, im not sure how this happens or how it can be fixed but im pretty sure the photographer often finds a way of restoring them, even the original camera raw files on the memory card have been corrupted before now and he still managed to restore them but he had to send the memory card away to a specialist to get it sorted and it was quite expensive!
What type of image is it and where did it come from? is it a photograph from a digital camera or is it a computer generated image or what type of image is it? htat might help sombody to recognise what the problem is!
im afraid thats all i can suggest really! unless somebody has actually edited the image and accidentally saved it and writted over the original but i doubt it and if the thumbnail shows the full image then it wont be that
somebody else on here will probably be able to make a better suggestion than me or know what you can do to restore it back to its former glory!
best of luck! Laura