As apcyberax says, this is in line with T&Cs. Webmail access is usually closed first. You can try to access the mailbox using an email client and your password just to backup your emails. But the whole mailbox is due for deletion at any time now.
I assume you are not a customer, therefore VM staff can only assist to speed up the deletion to protect your personal data.
Sorry to drag this out. Just for the avoidance of doubt, so the Forum staff know whether they need to assist, can you make it clear to us:
Are you a current broadband customer? Is the address showing as valid if you go to mailtester.com?
If the answer to BOTH questions is YES, the Forum Staff can investigate whether a Move & Transfer is possible. This is an internal process for the IT department, and would link the orphaned address and mailbox to your billing account. You would lose any addresses and mailboxes currently linked to the billing account.
(If the answer to either question is NO, then they cannot help, but this advice may help other visitors to the Forum in future.)
I also have not been able to gain access to my virgin.net email account for three weeks now. I never received an email about this shutting down. I am still a VirginMedia customer as I pay direct debits every month for the broadband. I have had that email address for 20 years and want to keep it. I am self employed and rely on that email address for business. This is insane. I've called the Virgin helpline multiple times and have never had an answer. I was most recently told that my email address had been changed but they wouldn't give me my new address so I could log in. Please help. This is very distressing