You are using graphical or command line interface?
If it is graphical interface, try to edit /etc/inittab to runlevel 3. And you can login the system by using command line interface. And using startx to start X Windows!
If you still have same problem under command line interface. please check
this
http://publib16.boulder.ibm.com/pseries/en_US/files/aixfiles/inittab.htm
and
http://www-2.cs.cmu.edu/~brians/comp/bbs/Ifmail-HOWTO-6.html. Which may related to your problem.
Normally this happens because of your /etc/inittab, or getty file.
And another possible problem is the /etc/passwd file. Check that file and make sure your shell is not /dev/null or /bin/false.
And by the way, next time you better describe your problems more clearly!