Macbook Pro stopped syncing photos to and from iCloud suddenly
Here are some solutions found on this Link


  1) Check the Internet connection on your devices, As you may know, a
  stable network connection on your iOS device and Mac is required to
  upload photos to iCloud. Thus, you need to make sure the Wi-Fi network
  connection works properly on your Mac and your iPhone (or other iOS
  devices). If you find Wi-Fi not working on your devices, learn how to
  fix it from the related article.
  
  This is the first solution you should try when iCloud photos do not
  sync to Mac or photos are not downloading from iCloud to Mac.
  
  2) Check your Apple ID To sync photos across your Mac and iOS devices,
  you have to sign in to iCloud on these devices with the same Apple ID.
  When you find iCloud photos not syncing to your Mac, go to check your
  Apple ID and see whether it’s the cause of the issue.
  
  3) Turn on iCloud Photo Library and My Photo Stream To access photos
  your photos and videos from all your devices, you need to turn on
  iCloud Photo Library/My Photo Stream on both your iOS device and
  computer. If iCloud photos are not syncing to your Mac, you should
  also check whether the options are set up correctly.
  
  On iPhone/iPad: Go to Settings > [your name] > iCloud > Photos and
  then switch on iCloud Photo Library and My Photo Stream. On Mac: Go to
  System Preferences > iCloud, make sure Photos is selected and click
  Options next to it. Then turn on iCloud Photo Library and My Photo
  Stream.
  
  4) Check your iCloud Storage Apple provides 5GB of iCloud space for
  free. But 5GB is generally not enough for you to store your iPhone
  photos and videos. It's possible that iCloud Photos Not Syncing to Mac
  is due to insufficient storage in iCloud. If that’s the case, you can
  either buy more iCloud storage or go to Apple Menu > System
  Preferences > iCloud to manage iCloud space on your Mac.


I hope this helped fix your macbook pro.