If you find the login window no longer lets you log in, what do you do?
You can boot to the install disk and reset the password for the user.
Ok, what if that does not work?
If you have another OS X server online that is doing Open Directory, try firing up the Server Administrator tool on the server you can log in to and do this:
Server Administrator -> Select the server that is in trouble click the "Access" button Click on Services Select the "Login Window" service Make sure you allow access to the users and groups you need, or allow all