Photos library not syncing with iCloud anymore on my iMac

I recently discovered that the Photos Library on my 2019 iMac 21.5" 3 GHz 6-Core Intel Core i5 running Sequoia 15.6.1 (24G90) is no longer synching with my iCloud Photos. I followed the instructions to quit the Photos app and relaunch it while holding down the Option key. When I do I get a dialog box instructing me to choose a photo library but there's only library one available. Clicking on that library opens Photos with a half dozen recent photos but no older ones. No new photos have been synching since I checked a few days ago.

All my photos going back to March 2004 are intact and accessible when I log into iCloud through my Safari browser.

All my photos are also accessible through the Photos app on my iPhone 14 running iOS 18.6.2.

Thoughts?

[Re-Titled by Moderator]

Original Title: Synching Photo Library with iCloud