A family of Microsoft word processing software products for creating web, email, and print documents.
Are you saving to OneDrive? This could be a temporary server issue, in which cases it should resolve itself after a few hours.
If you need further troubleshooting, see https://support.microsoft.com/en-us/onedrive/fix-onedrive-sync-problems-on-mac .
EDIT: There is also an ongoing issue with several of Microsoft's services, which may or may not be relevant to the symptoms you encountered. See the service health status page at https://status.cloud.microsoft/ .