Maybe a problem with the Previews.
Some suggest deleting the previews folder entirely (using Explorer/Finder) and let LR automatically re-build previews when LR is re-started.
Edit: DJ just beat me to this answer!
You could try going back to Library grid, select all photos in the folder, and rebuild previews from the menu-
Menu>Library>Peviews>Build 1:1 Previews.