Taken from the F.A.Q
Q. I just upgraded to a new version of vBulletin and now my index page is blank or templates are missing!
A. Upgrading to a new version of vBulletin will remove any templates that were created for mYvBindex with the exception of any that you have customized. To re-install the templates, upload the myvb_install.php file to your forums/admin folder and go to it in your browser. Then select the option to install just the templates.