Check that the Caps-Lock key isn't on. Double check that the username was correctly typed. Or have you forgotten your password?
Have you recently modified your .xsession, .profile or other login files?
It's possible you made a mistake...
When logging into a CSE linux machine, you should have a "Sessions" menu available. Try selecting a different one. The "Failsafe" option should be the most minimal and safest. Use this to fix any errors in your normal login files.
Also, if you recently changed your shell, you may have set it to something silly like /usr/local/etc/logins/tcsh, instead of /usr/local/bin/tcsh. The symptom of this is the system hangs on 'validating login'.
Which computer are you logging into?
Not everyone has permission to login everywhere. Try logging into another host.
Did you try to login to www.cse.unsw.edu.au? This is the web server. You should instead use login.cse.unsw.edu.au.