So you were able to login to it before? You can change your email via the user panel, though I've not tried it yet so I'm unsure if it requires you to know your old account.
I'd assume as long as you know your password you can change it no problem, although users are literally unable to hack into your account unless you gave out your password.
Your old account may have possibly gotten deleted if it was inactive, staff have been doing that with most old and inactive accounts. You can always PM a staffer about this.
Hope it works out for ye, but you can change you username VIA User Panel. ^-^