When was the last time you had signed into this account to check your email? This kind of message is typically seen when someone closes their account and then realizes they needed it after all and then tried to sign into their email again. Would that apply to your situation?
You mentioned trying to contact Yahoo for the last two months without getting a response, what contact method were you using?
Being that Yahoo doesn't offer customer service by email I can understand why you haven't gotten a response.
One way an account can be closed would be due to inactivity. If it's been more than 12 months since you last signed into this account that would do it. Other way an account could be closed would be if a TOS violation occurred.
Are you posting this question from the account having this issue? In order to know if you can get your account back first I'd need to know when was the last time you had sucessfully signed into this account to check your email? Are you able to go to the Yahoo home page and click on the "Sign in" button and sign into this account? If so do you get the error message when you click on the Mail button?