I am also one of the thousands of people suffering from the 'Mailbox Unavailable' message when I try to check my @ntlworld.com email via the Virgin Media sign in page and emails are also not coming through in Outlook.
I have already changed my account & email passwords both on the Virgin Media sign in page & in Outlook but this has not fixed the problem. This has been going on for over 2 weeks now.
Thanks for your post and welcome to the community forums. I'll PM you shortly, to get your account and mailbox details. If it is an email that is not linked to an active Virgin Media account then there will not be an option to reactive the account.