The most common reason for not being able to log in to your admin panel, even though using the correct login details, is in the Cookie Domain and Cookie Path values.

The solution is simple - find the tools.php script in the do_not_upload folder of your vBulletin package and upload it to your admincp directory.

Then execute it from your browser, i.e. :

www.yourdomain.com/forum/admincp/tools.php

Use this script to reset the Cookie Domain and Cookie Path back to their default values.

Please note that in order for the tools.php script to work, the install folder should exist.