Hopefully you found this page from a forum search and not because someone had to point you to it!!!
Sometimes, due to a PHP or other 500 (internal server error) error, a Joomla component such as VirtueMart can stop working and just show a blank page. A lack of display hides the relevant error message, or you may see Joomla's 500 error page.
When this happens check the server error logs (not access logs) if you have access to them. The error message will usually show what the problem was. Often it is simply the PHP memory limit on the server is set too low (min 128Mb, pref 256Mb)
You can also enable the vmdebug in Configuration/Shop - enable debugging messages. It suppresses the fatal errors and allows debugging in many cases. VirtueMart itself has a logging option. Furthermore virtuemart can disable the display of php warnings & notices to prevent perceived errors due to harmless php notes.
See this tutorial for more diagnostic options: http://www.ostraining.com/blog/joomla/white-screen-of-death