Login Failed message on iCloud account on MBP13

I'm getting a Long Failed message on my iCloud account and can't send mail from it. The Inbox and all my mail folders are accessible. Just sent an email from the account but the Login Failed message is still there.

Rebooted the MPB13 and no change. This appeared after sending an email, which failed to send, but sent successfully from another account I have.

I guess the question is, how do I get rid of the message?