If it is a secondary My VM account you're having problems with then you can just sign in to your primary My VM account -> My Profile -> Manage accounts -> click view/change next to the account in question and password change it.
If it is the primary My VM account you're having issues with: Contact customer services and have them do a password change. Either by ringing 150 from a VM phone / 0345 454 1111 from any phone -> Option 1 -> Confirm your details-> Option 2 (Faults) -> Report a broadband fault (covers all My VM sign in issues.) Ask them to do the password change on the account in question.
Or ask via the Webchat from a laptop or desktop. Between 8AM to 8PM M-F and 9AM to 5PM Sa: Account and Billing -> then the top My VM sign issues option -> Chat online. Reload the page every 3 minutes if it says the agents are busy. Do this repeatedly until you get through to an agent. Ask them to do a password change.
Once you've got a new password, remember to change the security question when you get signed in. And change the password you were given to something different too.
______ Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.