Apparently the original files are just gone, apparently they had digital backups of all their movies and their backups failed. Pixar also keep their original digital files for the first two Toy Story films and just re-rendered the film at 2K / 1080p for Blu-Ray which was a smart move IMO.
Toy Story was rendered at lower resolution for 35mm though, so it would be good to scan it to have to proper render.