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. The best thing to do in this case is to sign completely out of our member interface and log in again from scratch.
If this happens for some other reason, try accessing your database from the command line or web site. If that works, try closing your browser, or, as a last resort, deleting any browser cookies you have from members.nearlyfreespeech.net.
If you are not able to access the process at all, treat it as a lost MySQL password.