Take a look in /etc/inittab .
You should see a line like:
That means your default runlevel is '5'
Somewhere below that you should see a line like:
That means that in runlevel 5, run '/etc/X11/prefdm -nodaemon', which is the graphical login.
That's from Fedora, so your distro may have a different runlevel set, or may run a different command to start the graphical login.