I'm very sorry to hear that your email account has been compromised.
If we detect that an account has been compromised we will usually reset the your password for all of your email & My Virgin Media accounts to lock you out. We’ll then text you and advise you to go online and reset your passwords as necessary.
I'm unsure why you were able to log in as normal on this occasion. Did you log in directly, or via a mail-client such as Outlook?
Sorry for resurrecting this thread again. I am in the same boat. Received a test message from VM on 26 May, and again today (2 Jun) advising that my Virgin Media mail account may have been used to send spam, and that my account has been locked.
However, I can sign into my VM account using my current password. I just brushed it off as spam itself.
Strange that this all has happened following the security breach a few months back.