Suddenly my Mail app won't connect to yahoo server

I've been using Mail for years on my MacBook but this past week it hasn't been downloading new messages from the Yahoo server. When I look in the "Connection Doctor" window, all the radio buttons on the left go green, except this one...

That little wheel just keeps going around and won't go green. When I try and check the account in Mail prefs, I eventually get to this warning window...

I don't seem to be able to find any way of re-entering any details, or, as it says, 'verifying the settings', when I'm in Mail prefs. Anyway, I haven't changed anything, it's simply started doing this. I can download my mail OK on my iPad, but not on the MacBook. I'm using Mail version 9.3 BTW.

Help someone?