When the VM National service was closed down in late 2014/early 2015, VM customers had the option of transferring their broadband services to Talk Talk or to another ISP.
If they moved to Talk Talk their legacy email account remained available for 12 months, if they moved to another ISP the email account would be available for 30 days. After these periods the email accounts should have been closed.
There was also the choice of moving to the VM cable service (if available) where it was possible for a "move and transfer" to associate the email accounts to the new cable account. This would retain access to the legacy virgin.net email addresses, in perpetuity, on the new virginmedia.com account.
Whatever option was chosen, at all times Virgin Media retain control of the administration of the virgin.net email service. Talk Talk do not, and never have had anything to do with it.
If neither you nor Virgin Media can see your legacy virgin.net email address on your account then it's likely been classed as "an orphaned account" https://community.virginmedia.com/t5/Email/Orphan-Email-accounts/td-p/3492073 and should have been closed after 12 months, or 30 days, depending on the circumstances.
As noted earlier, the account holder themselves will have to contact Virgin Media about this issue.
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