Photos stopped uploading to iCloud. What can I do?
The surest way to resolve this is to create a fresh Photos Library by following these steps:

⌘ cmd-Q out of Photos.
Go into  → System Preferences → iCloud → Photos Options… and uncheck iCloud Photo Library.

Reboot if you want to be safe, though it shouldn’t be necessary.
Launch Photos with ⌥ opt-click.
Click Create New… and name your new Photos Library.

Go into Photos → Preferences → General and click Use as System Photo Library.

Go into Photos → Preferences → iCloud and check iCloud Photo Library.
Wait for your photos to be redownloaded from iCloud.

If you’d rather try to keep your existing Photos Library, follow these steps instead:

⌘ cmd-Q out of Photos.
Create a backup of your Photos Library.photoslibrary file.
Launch Photos with ⌘ cmd-⌥ opt-click.
Click Repair.

Wait for macOS to repair your Photos Library. A full sync should take place once the repair has completed.

Some screenshots courtesy of OS X Daily.