Picture greyed out in Aperture?!

I hope someone can help me here. I am on Aperture 3.5.1. I am combing through my library and noticing that SOME/selected photos are "greyed" out - please see attached screen shot. When I double click those images, they are temporarily restored to 'normal' but the issue is that when I export to Blurb, they remain "greyed out". None of my files are referenced.

Has this happened to anyone? How can I fix this?

In the attached image, one on left is 'normal'; one on right has they 'greyed out' problem.

Thanks