Unfortunately, some virgin.net users have found their mailboxes closed with no notice at all which is unacceptable IMO.
You should get a reply from a VM forum team person on here, usually within a few days. It is likely they will tell you nothing can be done.
When a mailbox is closed, the process seems to be that it enters a 'locked' condition to begin with, where it still exists but you cannot access it. Some time after that it is permanently deleted, after which point it is gone and cannot be recovered.
I have read a couple of posts on here where some users in a similar situation to yourself have made some progress in different ways. One route which regained temporary access for a data rescue mission is here
(although the post originally advised to contact the VM data protection officer DPO as well as the ICO) but has now been edited by VM to raise a DSAR which will be useless in as far as recovering electronic data in a useful format. You need to regain access to the actual mailbox to recover data. In any event, speak to the ICO if for no other reason than to raise awareness with them of what VM is doing and get their advice on the legality of deleting your long-held mailbox with no warning.
Another method is mentioned here is by contacting the executive team
https://community.virginmedia.com/t5/Email/EMAIL-ACCOUNT-CLOSURE-RESOLUTION-virgin-net/td-p/5589588
where you could complain and request temporary access is restored.
If you do manage to regain access, there is a post here
on a possible process for data collection and migration.
An archive post below describes a method for data archiving
When you try to connect any email software to the mailbox you are likely to need an app password and that is mentioned here
https://www.virginmedia.com/help/broadband/manage-email-settings
This seems to be a situation where you very much create your own luck in so far as regaining access is concerned and both of the above routes are subject to the random outcomes of dealing with VM so you may not get the same results.
Good luck, I hope you can regain access to recover your data.
