loss of email service

When the technician set up my email service with Comcast, I was assured that I could continue to use my att.net email address. That has worked for a few years now, but apparently the switch to Yahoo has cut me off from all email. The info you have on how to update my email (Apple Mail) doesn't match anything my computer is showing me, so rather than doing anything that might cause problems to get worse, I'm hoping you can tell me how to get my email--and, since hundreds of people have the att.net address, I want to get mail sent to that address. Thank you for any guidance. (I never did get a notice from you about the intended change -- but perhaps that is because of the att.net address, as well.) HELP.