This is usually caused by one of two things. Either there is suspicious activity which has caused VM to lock the email address, for your security, or you are not a customer/have an orphan address and the mailbox is approaching deletion.
So as you have been able to change the password, that points to you being a customer. As long as the email address is linked to your billing account, then the Forum Team (VM employees) are the next step. They can get the mailbox unlocked. They will reach this thread in the next day or two and get your security details by private message.