My Just noticed my photos have stopped uploading to iCloud

Is the 'Optimize Mac Storage' feature turned on in your iCloud Photo Library settings?

Thats not true as iCloud says I have 80gb free. It's as if it wants to grab all my photos and upload them again.

That is what will happen, if you turn off iCloud Photo Library and enable it again. Photos will require enough cloud storage to upload all photos in your library in addition to the the photos already in iCloud. That is because Photos has no way to tell, if the Photos already in iCloud are the same you are trying to upload.

Your 45000 photos and videos will probably need at least 200GB of iCloud storage. My iCloud photo library with 45000 photos has a size of 210GB on the Mac 166GB in iCloud.

Either remove other items from iCloud Drive to free storage temporarily or sign up for more storage for a month to be able to upload the library. Then you can cancel the additional iCloud storage again.

I have exactly the same problem as stated by kiwijohnny. I did not change anything myself but following the Sierra upgrade I experienced the same issues. This is not a good Apple experience I must say...

This is not a good Apple experience I must say...

Photos is using the worst case estimate of the additional storage required in iCloud - the size to store all photos. The alternative would be to compare each´h and every photo to the photos in iCloud Photo Library - that would require to upload all photos for comparison, and might take a week or more. You would not like to have to wait this long, before you can enable iCloud Photo Library either.

There is a second option you could try to enable iCloud Photo Library again. Instead of uploading your current local library, move it to an external drive and create a new, empty library. Enable this empty library as the iCloud Library. Now wait for all photos to download from iCloud. You need only to export your new photos from your old library and to import them to your new library. This will save you from having to pay for a month's subscription for temporarily more iCloud storage. But there are a few drawbacks:

• iCloud PhotoLibrary does not store the People album. You will have to name the faces again.
• The projects (books, calendars, cards, slideshows) will be missing, so you need to keep the old version of your library for these items.
• The new library will cause Time Machine to make a full backup of the library, because it is completely new.

Thats the thing, I never turned it off.

The upgrade to Sierra may have done that. I found iCloud Photo Library disabled in the System Preferences > iCloud > Photos after upgrading to Sierra, but it remained enabled in Photos > Preferences > iCloud.

The upload continued to work, but I could not enable it in System Preferences > iCloud > Photos. I upgraded the Storage plan to 1TB, and then I could enable iCloud Photo Library in the System Preferences as well.

The weird thing was, that the iCloud sync for my Photos Library never stopped working, because iCloud was still enabled in the Photos Preferences.

it should work, if you upgrade your storage to allow the upload, and then downgrade again, once the library has been synced.

Good Luck 🙂