Sometimes, due to a PHP or other error, a Joomla component such as VirtueMart can stop working and just show a blank page. This lack of display hides the relevant error message.
When this happens check the server error logs (not access logs) if you have access to them. The error message will show what the problem was.
Or see this tutorial: http://www.ostraining.com/blog/joomla/white-screen-of-death