How are you trying to connect, console, telnet, ssh?
This maybe due to restrictions on root loggin, or is that you don't know/remind the root's password?
If you've forgotten root's password you must enter into maintenance mode, starting from a mksysb image or base installation media (installation CDs). The media your starting from must be at same oslevel as the server, so if you've updated it since installation you may not be able to change root's password.
Once you've started from media, you may be able to enter a shell and change root's password.