If that is supposed to be a clarification of the original problem statement, it isn't working.
My response does not change, much. I've never seen evidence of cancellation for non-use. What I have seen is "phishing" e-mail, claiming to be from AT&T (but actually not) stating that idle e-mail accounts will be canceled, unless the subscriber "proves" they still use the accounts by providing the login details (username+password). These are not from AT&T; no service provider will send requests for login details out of the blue.
If your login no longer works, have you, maybe, responded to such a request? Along with the noted security lapse by Yahoo! Voice (I believe it was), this would account for a login failure.
I will reiterate: If your account has actually been compromised, the hijacker very likely will change the login, causing the failure you see. I have a relative who had this happen; probably due to a brute-forced weak password.
I strongly recommend you go to »
mail.yahoo.com and try this:
Password reset request.
If that does not work, you will need to contact Yahoo! support for further assistance.
If you think my advice is off base, you will need to improve your problem statement; as stated (twice, so far) it is too ambiguous.