Hi there and welcome
The email about yahoo cancelling your account was a fake email, not sent by Yahoo or AT&T. This has resulted in your account being hacked as they gained your username and password.
We need to try to get you back access from this asap.
First, tell me your yahoo address with the problem please.
I can see you are a first time user to the site. What we do here is either a question and answer type of conversation or remote support where I would actually connect to the computer and have a look and attempt to fix the problem for you.
Remote support can save a lot of time if you are not comfortable performing repairs yourself. Sometimes even with instructions you can make a mistake and make things worse.
I will send an offer through for remote support and should you accept I will connect and see what is going on and fix the issue. Otherwise we can continue to talk like this but I will need to ask a number of other questions and try and walk you through some steps.
Both options are 100% guaranteed by the way. Remote is just quicker and avoids the risk of you making a mistake and doing something wrong.
If you want remote click get extra help. If not simply reply to me.