Unfortunately, you can't - once you leave VM, your email accounts are supposed to be disabled and deleted after 90 days or so but often this doesn't seem to happen and they linger on in a sort of zombie-like existence for ages until some kind of process at VM starts a clean-up exercise and they get deleted.
Now how are you trying to access the mailbox, via webmail or using an email client? When a mailbox is in the process of being cleaned up, webmail access is often the first to go and in that case you might still gain access by setting up and email client (Thunderbird or Outlook etc.) and at leats be able to copy any emails you want to keep before it goes completely.
The only way you would be able to change the password is by logging into your myvirginmedia account, which you can't do as you're not a customer and don't have one. The VM staff won't be able to help you with this either other than making sure that the account and mailbox is properly deleted.
