[Kirk Y]

Have you installed any modifications recently or have you made any changes to your vBulletin Options - specifically in regard to your HTTP Header and Cookie Options?

In your 'includes/config.php' file, place the following on its own line and try to login afterwards:

define('DISABLE_HOOKS', true);

If (and only if) that does not work: upload the tools.php file, contained in the do_not_upload directory of the vBulletin Package from the vB.com Member's Area, to your Forum's AdminCP directory and run it. From there, choose to "Reset Cookie Domain" and "Reset Cookie Path".