iCloud and Outlook for Mac

For some reason, all of a sudden, I cannot receive my iCloud account email in Outlook for Mac, although I can do so on my iPhone and iPad.  I get the message, "Mail could not be received at this time.  The server for the account iCloud returned the error 'Authentication Failed'.  Your username/password or security settings may be incorrect.  Would you like to try re-entering your password?"

My settings have not changed, so I'm puzzled as to why this is happening.  If anyone has any advice, it would be most appreciated.