Help - lost Mail link

Hi, I'm 70 and not too good with all this so be gentle, please!

After working quite happily for several years, Mail on my iMac can't link with my primary email server. I'm on Sierra 10.12.6 and can neither receive nor send messages through Mail and there are no Mailboxes. I'm not aware of any changes which may have affected this.

In Mail preferences, my email account - Yahoo (btinternet.com) - is shown as offline - "no preferred outgoing mail server". There are no settings in "account information" and Connection Doctor merely says "account failed" and "verify username and password" both of which are fine. I have tried to delete and add the account back on - also without success. "Take all accounts online" has no effect either.

Email is working fine on my iPad and iPhone. My btinternet.com account is also working OK - just not with Mail!

Has anyone got any ideas - hopefully which I am able to understand?

Thanks

Bill