How are you accessing the mailbox? Is it directly from the site or via email client/app? If you are attempting to access from our site, have you been able to try the 'forgot username' and/or the 'forgot password' functions? Do you have a screengrab of the error?
Mailbox not available is usually something from the back end of the service. I'll PM you shortly to get your account details. Just to clarify, if there isn't an active VM account attached to the mailbox, I can only delete the mailbox.