[Lynne]

My guess is that you did not change your cookie domain. 1. Download a copy of the vBulletin zip file from vBulletin.com (if you don't already have it).
2. Once you have done so, extract the contents and navigate to the do_not_upload folder, from this folder select the file named tools.php and upload this to your /admincp/ directory on your board (via. FTP).
3. Type in the path to the file in your address bar, e.g. http://www.yourboard.com/forums/admincp/tools.php
4. Enter the details required.
5. You will then be presented with a list of options, you'll need to look for one which says 'Reset Cookie Domain' (or something like that - there are two settings, reset them both) and follow the on screen instructions.

Once this has been done, remove tools.php from your AdminCP folder.