Welcome to the forum@) I'm sorry to hear of the problems that you have been experiencing with the Virgin Media website. There have been some issues with the VM website last week. I'll send you a PM (purple envelope top right corner) for some additional info to help us locate your account and see if this problem is affecting you.
This is an ongoing problem. Paying through the VM billing page takes aprox 15 mins. I have been doing this on and off for over a year and often give up and use the phone. Its not a temp problem its permanent and a total disgrace for a communications company.