Yahoo takes your account's privacy and security very seriously. As a security measure, Yahoo will only let your reclaim an account if you can definitively prove you're the rightful owner.
Since Yahoo does not collect much information up-front when you register, there are limited options for verifying that you own an account. Your best option to avoid losing access to your account is to make sure that your account recovery information always stays up-to-date.
When the Password Helper says that your password can't be reset online - If you've tried using the Sign-in Helper and weren't able to get access to your account, other recovery options may not be available. We know it can be frustrating to see the fail page, but in these rare cases, we recommend creating a new Yahoo account.