The important bit is "are you or your wife still a paying customer"? If not, your mailbox may have been deleted. VM has recently been purging mailboxes of non paying ex-users. In any case, ex-users mailboxes are supposed to be deleted after 90 days of leaving.
However, if she is still a paying customer, try following these steps to see if a password reset allows access:
Sign in to My Virgin Media with her user details
Go to Account settings, then Account details
Tap Manage next to Manage your Virgin Media Mail app password under Virgin Media Mail
Then, under Virgin Media Mail app password tap Get password
Continue the flow and a new secure password will be generated, and the mailbox unlocked
Update any third-party apps that may be use to access your Virgin Media Mail emails with this new app password.