This often happens when you log in to phpMyAdmin with your administrator username and change the password for that username while you're logged in.
Try closing your browser, or, as a last resort, deleting any browser cookies you have from phpmyadmin.nearlyfreespeech.net.
If this happens for some other reason, try accessing your database from the command line or website.
If you are not able to access the process at all, treat it as a lost MySQL password.