A Microsoft file hosting and synchronization service.
Hi Dermot,
Thanks for your query in this community.
Per your description, I would like to confirm if this is the scenario you are currently encountering with:
Your co-worker has shared a folder in his OneDrive for Business to you, and you will find it under the tab **"Shared with me"**in your OneDrive web page.
See files shared with you in OneDrive
And you synced this folder via the OneDrive sync client and keep collaboration with your co-worker.
Somehow, the syncing in the folder has been stop and when you check the folder in your co-worker's account, it's marked as Private.
In this case, most likely the link has been expired, by default, when your co-worker share this folder to you, he might unconsciously set up an expiration date for this sharing link. To around this issue, please ask him to share this folder to you again, this time, do not set an expiration date. And you should be able to work on this shared folder again.
If this method didn't help or there is any misunderstanding on your question, please feel free to point out. Meanwhile, if it's convenient for you, please provide us with some screenshots on the issue, so we can take a further investigation on this issue.
Regards,
Alex Chen