Surprise! Without doing anything, about an hour later after I had the problem, it all started working again...receiving and sending emails.
Interimly, I ued the iCloud webmail.
And in response to your question...I had highlighted a User Account in the left hand column but it still would not let me enter anything in the incoming server space.
Mysteriously... who knows why, it all is working again.
Thank you for your reply. Terry