If the mail address in question is the one you have used for your forum mail address, then this mail address has been purged from our systems as it is linked to a disconnected ADSL account. The mail address can be ported through to your cable broadband account but this would require a call to our support line as security will need to be cleared on both the old and the new accounts before this address can be ported across.
This is the same problem I am experiencing. We use three different virgin.net addresses and for a while I could get into none of them, a few days ago I managed to access all three for a while, yesterday I could only get into the primary one, and today so far, none! Yet, all three work on my newly acquired ipod (which I don't really know how to use properly yet).
I had to register for this forum using a none virgin email account as I was unable to access the confirmation email when I first tried to register using my virgin.net email. I did eventually manage this, so maybe, if I can manage to login , I will add another post to this thread using that login and see if any customer services help person can come up with any further help from that.