Where did the virgin.net email address originally come from? Did you ever have a dial-up internet service?
The bad news however is that use of any of the Virgin Media email provision is dependent on you having your broadband (at least) with them, once you leave (the case of the virgin.net addresses is a little more complex but the same applies) then the mailbox should be deleted after a couple of months. Sometimes this doesn't happen and the mailbox staggers on in a sort of zombie state but eventually VM's clean-up processes catch up with these 'orphaned mailboxes' and they get deleted.
It's highly likely that yours has, or in in the process of, been deleted - a member of the forum team may offer to contact you to see what could be done but I really wouldn't hold out much hope.
