iPhoto only opens thumbnails

When I open iPhoto I have my list of thumbnails. When I click on any of them it opens to a blank screen and the magnifier is all the way to the left. When I move it to the right a thumbnail appears and a selection box appears on the thumbnail and gets smaller leaving the portion within the box visible and it greys out the rest of it.

I have made no changes to iphoto since my last import 2 days ago when all photos were opening as normal. They were also opening normally after the import. This just started happening this afternoon.

Any help would be greatly appreciated.

Re-launch iPhoto holding down the option and command keys. A list of diagnostic/repair options will appear. Check ALL of them, then go make a cup of tea while it does its thing (may take some time).

If you haven't been mucking about with the iPhoto database file, all should be good after that.