You can disable logging in to fix it up first. Make sure you have at least .htaccess authentication up though.
In /admincp/global.php, find:
PHP Code:
// ############################################ Start Login Check ####################################
Remove everything in between that and:
PHP Code:
if (file_exists(DIR . '/includes/version_vbulletin.php'))
Make sure you reupload global.php afterwards.