You have to be a current VM broadband customer to be able to continue using VM email.
VM has been deleting mailboxes which are not associated with a current VM broadband account
https://community.virginmedia.com/blog/Digital-life/some-important-information-if-you-have-a-virgin-media-email-account/5607207
Some people, who are VM broadband customers, have found that their mailboxes have disappeared because their email address has become 'orphaned' from their broadband account. In particular this has affected virgin.net mailboxes (as well as other VM legacy domains).
Do any of the above circumstances maybe explain your situation?