What exactly do you mean by "nothing happens"?
Did you enter user/pass and the login failed or couldn't you enter anything at all?
If there are problems with the gui you can always press ctrl and alt and F3 at the same time to get the command line. From there log in as the root user. If it is a password issue you can set a new password for the user there. Type:
Press ctrl+F7 to get back to the gui and try to log in again.