In case you are one of the select few who are having issues with the dreaded wp-admin login and clearing your cookies didn’t work. I would recommend reinstalling like several others, but is seems a tad overkill.
The key seems to be the details in the configuration that are hidden in the admin protected area such as siteurl. I was able to effectively break the login by adjusting 1 but not both of the admin settings when I moved my site from the test domain to the live. Upon using a database query I was able to rectify the offending database record in wp_options.