Suddenly I cannot import photos into Lightroom. When I try to import the source directory displays correctly in the left panel but no photos previews display in the main window. I've tried using different SD cards, different drives and photos from different cameras. I downgraded from Lightroom 5.4 to 5.3 without any change. I can't remember for sure if I upgraded my operating system when the problem started but I don't think so. I am operating OS X 10.9.3.