Are you trying to log in as root to a GUI? That's often disabled; you'll need to set up a user account from the command line. Hit Alt+Ctrl+F1 to get to a login, log in as root and then set up a user account with
adduser. You should be able to log in with that account.
If that doesn't work (or is not the problem since you're not using root), something's botched; boot to Single User Mode (you can use this description:
http://www.cyberciti.biz/faq/grub-bo...gle-user-mode/) and use
passwd user to set a new password.
M.
