When you leave VM, VM is supposed to delete your mailbox after 90 days of you leaving. In the past VM has been very unreliable at doing this but in recent times it has been doubling down on doing so.
VM has also implemented changes to security, one of which is an app password for email client apps. To generate the app password you need to log into My Virgin Media to do so (which you won't be able to do if you are no longer a VM customer).
If you try a password reset now, you may end up getting locked out of your mailbox altogether.
As a matter of urgency, you should set about switching all of your email contacts and services which rely on that BY email address over to a new email address not linked to an ISP. You should do this while you still can access the BY mailbox as email security changes for some sites will send a confirmation to the old address before allowing you to set a new one.