This can be helpfull if you created / installed a plugin or product which has broken your forum and you are not able to access the AdminCP anymore.

To disable the plugin/hook system open your config.php which can be found in your forumroot/includes directory

Just below
define('DISABLE_HOOKS', true);
and save the file.

If you would like to enable the plugin/hook system again, either remove the line again or simply comment that line out. To do so, add two forward slashes " / " so it will look like

// define('DISABLE_HOOKS', true);
Next time you would like to disable the plugin/hook system again
you simply have to remove those two " / ".

If you are able to access your AdminCP go to

AdminCP -> vBulletin Options -> vBulletin Options -> Plugin/Hook System

And set it to No to disable.

Moderator Edit-

It is also important to note if you are using Windows operating system do not use the built in Windows Notepad to edit php files. This can add hidden characters that can break your site. Google for Notepad2 or Notepad++ (both free) or another code editor and use that to edit your php files.

If you already used Notepad and broke your site get a fresh copy of from your original download package and copy over the important information manually and use the fresh config.php file.