If there's not an ISP account linked to the Yahoo! account, the
only information that Yahoo! will have about an account holder is their name (which might be in their email address, so can't be used for security purposes), their current log-in details and the security question (such as a child's nickname) which was created when the account was set up. If you can't get in via here
https://edit.yahoo.com/forgotroot/
then the account is lost for all time.
A new (& better) email account can be created here:
http://www.gmx.co.uk/