Unfortunately, I don’t think this is going to be possible. Use of any of the VM branded email addresses is conditional on you have a live broadband account with them. Once you leave, all of the mailboxes should be deleted after 90 days although sometimes, as you have seen, this isn’t done on time and they linger on but are always prone to being deleted at any point.
I fear that this is what has happened to yours, it has either been deleted or has been locked in preparation to be deleted.
In the past, there were cases of the forum staff here intervening and managing to get the mailbox unlocked on a strictly temporary basis to give the customer a chance to copy out anything they want to keep, setup an alternative email address with another company and inform contacts of the change of address. However more recently, it seems that VM’s backend IT people will no longer do this for ex-customers, presumably a change in company policy.
I expect that a member of the forum team will get to this thread in a day or so and reply to either confirm this or to take further details from you to see if there is anything that can be done. But I do have to be honest and advise that the chances are not good.
