[Lynne]

Have you looked in your error_logs to see if something is there?

Also:
Create a new style with no parent:
Styles & Templates > Style Manager > Add New Style :
Parent Style: No Parent Style
Title: Default vBulletin
Allow User Selection: Yes
Save
Then browse the site using that totally default vbulletin style - do you still have the same problem?

If it is still happening - Try disabling your plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

PHP Code:

define('DISABLE_HOOKS', true);