Last time I had a similar problem when the updated driver for my graphics card didn't play nice with gnome.
You should try to boot into a single user non-graphics mode. One way to do that at the start of booting, tap a key when the grub screen appears, which will pause it. Edit your boot selection by entering the "e" key. Select the kernel command line and enter the number "3" at the end of the line. Then hit F10 (I think) to proceed with the boot.
That will boot you into init level 3 where you should get to the command line login. Try to start you X session manually from there "startx" and see what further messages you get at the terminal. Also, check your message logs in /var/log/*.
I know how frustrating that can be when you have work to do.