You need to be able to authenticate an account to unlink it or use it in Cross Save — Bungie cannot do this for you. I suggest contacting Microsoft to see if you can get the login information for that account. Your other option would be to create another bungie.net account and move the accounts you can access to that one. In doing so, you would lose any redeemed emblem codes and Bungie history on your current bungie.net account.