Same here since Tuesday morning. I did get through to someone on the phone on Tuesday who asked me to try basic things like use a different browser. He concluded it would be reported as an outage and would be fixed within 24 hours. Still not working. Scrolling through the Email section of the forum and can see this is a common problem atm but no communication from VM which is so frustrating.
Same issue here , I am an existing virgin customer and one of the secondary emails linked to my account has suddenly stopped working Tuesday. Getting the 'Mailbox not available ' on all devices, though I can sign in with the same account details into VM , just can't access the account and need it urgently for travel arrangements etc.