Pam Bush upgraded her Mac to the latest version of OS X, but her Photos library is full of black thumbnails, rather than tiny images. The Mac consultant who handled her upgrade told her it was a bug, and she needed to “rotate every image manually.” “Not going to happen in my lifetime,” Pam writes.
The consultant is technically correct: iPhoto and Photos rebuilds previews if you rotate an image, which can be done in Photos in the All Photos view by selecting Edit > Select All and then Image > Rotate Clockwise or Rotate Counterclockwise, and then the opposite after the rotation is complete.
However, for a large photo library, this is a lot of wasted time and effort, and might mask other problems in the file structure. Instead, you can rebuild thumbnails or repair the library, depending on the software you use.
Read more here:: Photo thumbnails on your Mac turned to black? Here's a fix