When was the last time you logged into your Yahoo Mail account? Does the Sign-in Helper recognize your account?
Yahoo put accounts in dormant if you aren't using it on regular basis and finally delete it if the account inactive more than an year. This was happened with me. However i recovered the email account from dormant but all of my email data removed. I have contact yahoo support but they said that my emails have deleted because of inactivity and can't be recovered. I suggest you to use "Sign-in Helper" on https://login.yahoo.com/account/challenge/session-expired?src=""oSrc&authMechanism=secondary&done=https%3A%2F%2Fwww.yahoo.com%2F to get your account back.
If you want to contact Yahoo you can use this url. https://www.yahoo.com/subscriptions/products/yahoo-plus-support-help