Neither fix will solve the mystery of why the mailboxes disappeared in the first place, but either could save you days/weeks of sleuthing....
I found the cause....the ISP. I am with Telus in Canada and, when I called their mail help-line, they advised they don't provide support for any third party products....not one...Apple, Samsung, Windows...none. They only provide support for their own platform, Telus Webmail.
I thought "Okay, fine. Let's have a look at webmail" and then it got disconcerting, very disconcerting. First, the technician couldn't do a search based on my Telus email address. Huh??? I had to think of specific names of emails that he could search for. Double huh???
Anyway, long story short, my case got booted upstairs where I may get my mail back in a week. I may, or I may not. I could not get a guarantee beyond "They usually find it". I was shocked.
It seems to me that, when a company takes possession of something as important and personal as a person's mail, they should absolutely be able to locate it, and they should be able to restore it in a timely fashion. POP is staring to look pretty good.
By-the-way is Abby Roade your avatar?
Yes, that's just over four years ago when she was three months old...her ears weren't even standing yet.