From reading many similar topics, it seems as if the first part of the mailbox deletion process involves putting the mailbox into a 'locked' condition. It still appears to receive emails but the mailbox owner cannot access the mailbox. In the second stage of deletion, the mailbox is removed entirely. From that point undeliverable messages are generated and the mailbox cannot be recovered.
VM's claims that it has contacted users before deletion are not reliable as many people have posted on here saying they never received any such notification. Without providing this notification, I don't believe it is fair, or reasonable, for VM to delete mailboxes which people have used for decades.
Unfortunately, VM does not behave in a reliable and consistent way towards those who have been affected by this. Some people posting on here have had an offer of help to see if the mailbox can be temporarily reactivated. Others have been told that nothing can be done.
What you do next will depend on your level of motivation, and need, to regain access to the mailbox.
One person dealt with the issue as below
https://community.virginmedia.com/discussions/email/email-account-closure-resolution-virgin-net/5589588
you will have to Google for the email addresses. They get removed if posted on here even though they are in the public domain. The one for the executive team is shown on the 'Accessibility' link at the bottom of the page using the 'Power of attorney ...' drop down.
In another topic the customer contacted the DPO
https://community.virginmedia.com/discussions/email/virgin-net-email-swindle-tell-bbc-rip-off-britain/5583100/replies/5586294#M281331
the post has since been moderated to say 'request a DSAR' but you should be requesting that temporary access is reinstated. The email address is in the privacy policy (again public info)
https://www.virginmedia.com/legal/privacy-policy
You could also raise the issue with the ICO and explain what has happened and seek further advice (unlikely to result in any useful outcome for you but they may take an interest if increasing numbers of people did this)
https://ico.org.uk/global/contact-us/contact-us-public/
Unfortunately, none of the above routes offer any guarantee of regaining access but you might want to try them depending on how badly you need to regain access. If you are locked out anyway, you have nothing to lose other than time/effort in trying.
If you have no need to regain access, you can request that VM permanently deletes the mailboxes. A VM person should reply here within a few days.