hmm..not sure, but have a look et /var/log and scroll through some files such as "errors", "XFree86.0.log" and "messages". It might be best to start up Gnome normally and wait until it stops, press CTRL+ALT+F2 (or any key from F3 to F6 which should be normal ttys) to get into a shell, log in and then do
cat /var/log/errors | less
and of course to other files I mentioned, or all in /var/log. scroll through and see especially the last lines in the file(s), if there is any sign of what has stopped and possibly why. I haven't used Gnome for a while now, so I can't remember exactly how to do these things..but give that a try and tell if you got anything that looks "interesting"
EDIT: of course, when you're in the shell and your Gnome is stopped/jammed, you can shoot it down like this: either give a command:
killall xinit
or, if that won't do, (you can be root if you feel like it)
killall -9 xinit
or if you don't want write that much, simply press CTRL+ALT+F7 (this should get you back to X which is stopped as you said?) and then press CTRL+ALT+BACKSCAPE and X should shut down and bring you either back to console or restart X. if you got back to console and want to restart X, simply give a command
startx
and that should do.