Downloading photos from icloud photo library is stuck
(Similar but not identical issues remain unanswered in other forums.)
When I try to open an existing project, a photo book, "Downloading from iCloud Photo Library" begins, but does not finish. The progress bar gets almost all the way and then hangs. It has been sitting that way for over 24 hours. There is no sound of the disk working.
I started this project/photo book several days ago, and this issue happened only when I tried to re-open it to continue working on it. I have 50GB of disk space, and no message about not enough space.
I considered turning off iCloud Photo sync, but the message "If you turn off iCloud library, all documents stored in iCloud will be deleted from this Mac" is confusing: If the photos I was using are on my Mac, why is it downloading from iCloud? I do not want to lose the project.
Why is Photos downloading from the iCloud Library?
How do I access the book project to continue work on it?
MacBook, OS X El Capitan (10.11.6), (Aluminum, Late 2008)