Thumbnails Disappeared

Most (but not all) the thumbnails in my Aperture 3 library have disappeared. Their location is marked by a rectangular dotted line. In some instances there is a warning triangle in the rectangle. If I click on the blank rectangle the picture will open.

I have tried selecting individual and all thumbmarks and running "generate thumbmarks". When I do this there is an indication at the bottom of the screen that Aperture is running but when the process stops, none of the missing thumbmarks has reappeared.

I was wondering if I should reinstall Aperture? I purchased it originally through the Apple Store so I suppose I can download it again. I would prefer to avoid doing this!