Photos not showing in viewer

Aperture version 3.6, OS X El Capitan version 10.11.4, 2.66 GHz INtel Core i%, 4GB memory 1067 MHz DDR3, storage capacity 1TG.

Photos show up in browser as thumbnails, but if I double click on a photo to open it, it does not show in the viewer (the screen is black, but if I move the cursor on the screen, the photo data is there, as I can see it move with the cursor). I tried to generate a preview by righ-clicking on the photo and holding down the option key, but it didn't fix the problem. I also tried repairing the database, repairing the permissions and rebuilding the database (by holding down the option and command keys while starting up Aperture). Any ideas what else I should try?