Cannot access my eMail through Yahoo.com

I have always accessed my AT&T (swbell.net) email account through Yahoo. For the past month, after I put in my email address, I get redirected to https://loginprodx.att.net, where I have to re-enter my email address, and enter the password. I am then re-directed back to Yahoo where I can access my Mail.

Except today... On redirect to https://loginprodx.att.net, after I enter my email address and password, I get the error message "Please make sure that your User ID and Password are both correct, and try again." The email address and password I entered are correct, after the first two failures I typed them onto a notepad, then copied and pasted them in, but still I'm getting that error message. I've closed out my browser, clearing the cache, tried again, same message.

I was worried that someone had hacked my email and changed the password, so I tried to signin to my att.com account, which is the same email and password, and I logged in successfully. Obviously, that is how I found this forum. I do not need instruction on how to reset my password, thanks...

1. Why is my yahoo.com signin now redirecting to https://loginprodx.att.net to signin?

2. Why does my email/password combination work to sign into my att.com account, but no longer work to sign into my yahoo/email account?

Please help, I really need to access my Primary email. I am writing this from my Secondary AT&T/Yahoo email account, which I am able to access today, so I will get a notice if you reply.