What causes a slide to appear blanked out? The top of the photo is visible but the rest is covered with a grey panel.

I have 3 slides that are suddenly appearing that way I don't know what I did to cause it. The slide appears that way in the media bin too -- even if I delete it from the media bin and replace it from the original source.

This is the second time I've seen this. What is happening and how do I reverse it?