A Microsoft file hosting and synchronization service.
Same here. Lost my shared folders in OneDrive on Windows 11 yesterday (22-Feb 2025). OneDrive started creating duplicate folders in the "FolderName-ComputerName" naming convention for the lost shared folders and stored some files I edited in there (presumably local copies of the online originals). The shared folders are still accessible (online in the web browser) via links shown in the OneDrive folder in Windows Explorer, but they are no longer accessible for syncing in OneDrive, and I can't directly view, open or save the files in any apps anymore.