A Microsoft file hosting and synchronization service.
Before attempting any resets, reinstallations, or changes to the OneDrive folder, I would like to examine the exact error to avoid risking those files. Could you please share a screenshot of the "A File Can't be Synced" message, including the filename and any additional details displayed before OneDrive closes? Also, if possible, please click the OneDrive cloud icon in the macOS menu bar and share a screenshot of its synchronization status or any errors shown there.
Additionally, please open Finder > Go > Go to Folder, enter ~/Library/CloudStorage/, and share a screenshot showing the OneDrive folder and its contents, if visible. This will help determine whether the files are still present locally but inaccessible because OneDrive is not running, or whether Finder is displaying an empty cloud storage location. Please also confirm whether you can see the older files when signing in at https://onedrive.live.com. For now, avoid deleting, moving, or resetting your OneDrive folder, since you mentioned having newer files that have not synchronized. Please hide any personal information in the screenshots.