How do I get my email to work (Windows Mail -- mine's Bellsouth.net)

I have not been able to get my email to work for four days—it keeps throwing up a Windows Security box asking for my user name and password (which, incidentally, have not changed in 15 years). The box always appears right before it downloads the email.

In the past, this has been a nuisance = sometimes a rather severe one, but, like constipation, would pass eventually. However, at five days, I am about ready to just delete the whole mess (if I knew how) and go with some other email source. I do not even know who to call or talk to about this problem. But, like my cell phone and other communications, I DO need my email (rather badly, I might say). Any help would be appreciated.