When you click unlock, it should come up with a dialog box to enter your password. In order to get this dialog, the application System Preferences is trying to access something called system.preferences.accounts. So something must be wrong when trying to access that, wherever it is kept.
It could be the permissions are set wrong on it in which case a disk utility repair permissions would be in order. It's odd that it would just do nothing and not report some error. Check the console (not terminal) application to see if there is any info in there when you try to authenticate.
I'm sure I had this problem once but I was messing with my admin account in Netinfo Manager and locked myself out. To get back in, I started up from the CD and chose to reset password from one of the menus. I would recommend that you do this because resetting the password should definitely give you your authentication back.