Temporarily boot to a lower runlevel. Runlevel 5 is X, runlevel 3 is command line and should be good enough, but runlevel 1 is maintenance where almost nothing will be started up. Here are instructions for Redhat on booting to a different run level:
Another option is to kick the boot into interactive mode when prompted to do so during boot up. You then have the option of not starting some services.
Clean up your space, and then try rebooting. Hopefully lack of space is the only problem, and X will then start. Otherwise, follow the previous advice on reconfiguring X.