hello, to which version did you update exactly? did you receive any errors during the update/installation? we located an issue in versions 248 and 249 which may cause a fatal error on virtuemart if you are not using the very latest 2.6.x release. this is fixed in version 250 (version 250 was tested on vm2.0.10 which makes it run on all version in between)
if you are running virtuemart 2.6.x, please enable VM debugging here:
which will show you the underlying Fatal error that causes this issue.
if you are running Virtuemart 2.0.x, please enable error reporting in joomla general configuration at the section server - set it to maximum.
best regards, stan, rupostel.com