I'm afraid that the chances are that the email address has now been or is in the process of being deleted.
If you take a read through this thread;
https://community.virginmedia.com/t5/Email/Email-locked/td-p/4957842
Which sounds like a very similar situation to yours, I have posted in a lot more detail (post 5) about the somewhat confusing history behind the virgin.net addresses. Bottom line, I'm afraid, is that you need to be a current VM broadband subscriber to keep using the email provision, otherwise it is likely to die at any moment without warning.