I have the same problem...I can't get any emails using Thunderbird and get the Mailbox unavailable message on Virgin home site, I can get some of my messages through Gmail and Hotmail which are both working fine. I phoned Virgin yesterday to no avail.
I get the following message form Thunderbird Sending of password for user .com did not succeed. Mail server pop3.virginmedia.com responded: Authentication failed. And on the Virgin Media site I get the Your Mailbox is unavailable message, I phoned VM yesterday and they tried to change the password and she couldn't so she gave me a ticket number.
An update...I tried to download from Virgin Newsgroups this morning and couldn't connect....is there anyone out there who can sort this out. I phoned Virgin on Wednesday and the woman I talked to couldn't change my password. I'm waiting for e mails from builders this couldn't have happened at a worse time.