Photos

I wonder what you've tried. Can you open the picture in preview by selecting it and hitting the space bar? Does Edit work? Have you tried quitting the Photos app and re-launching it? Have you tried re-starting your computer?

You should also try re-starting your computer in Safe Mode:

           Start up your Mac in safe mode - Apple Support 

If none of that has worked, please get back to us with more information. Do you use iCloud? Do you get an error message? Can you use File>Export>Export Unmodified Original to export a picture?

Let us know....