Cannot send mail

Hi,

Hope someone can help with this. I have just tried to reply to an email, on my icloud address, but the mail would not send. Instead I got a message on the screen saying “ Cannot send Mail, The username or password for “icloud” is incorrect.” I have never seen this message before or had this issue. I never sign out of my account so have not been trying to sign in. The email I was trying to send was part of a chain so I have been emailing back and forth without issues up to now. Also tried to send new mail to someone else but got same message.

Have no idea what to do.

Thank you

Steph