I tried everything that was suggested by Apple Support, nothing worked. I then read somewhere that the Photos Library only works now from the Pictures folder. Because of the size of my library I had stored it on an external hard drive, this worked up until recently. I wonder why, hmmn?
Finally I did this.
- Restart the Mac
- Connect it directly to your router (WIFI is probably too unstable)
- Copy the Photos library (Photos Library.photoslibrary) from its current folder to the Pictures Folder
- The copy might end with an error saying that the file already exists and leaves an incomplete copy in the Pictures folder.
- Check that the file size is as close as you can get to the original Photos library (it might not recover all)
- Duplicate the incomplete copy in the Photos Library this will create a copy you can use.
- Open Photos using the duplicated version you should now be able to see all of your photos in their Albums, etc.
- If you are satisfied remove the incomplete copy as you can't use this anyway.
I hope this works for you.