Some people report having troubles with photos disappearing from iPhoto's display pane, even when the files are still present in the iPhoto Library folder (see “iPhoto Directory Structure” in Chapter 2, “Importing and Managing Photos”). Try the following procedure to fix the problem.
But don't follow this process unless absolutely necessary, since it will cause you to lose all your film rolls, albums, keywords, titles, and so on. You end up with your photos, and that's what's important.