[Lynne]

Look in your error_logs (if you don't know where they are, ask your host) to see if there is a clue in there.

Try disabling your modifications/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); 


Don't forget that for vbseo, disabling the modification means to also get rid of anything extra you added to your htaccess file.