Here is what worked for me after X kept crashing upon reboots. If you only see a blank screen with no input, try hitting F1, F2, etc. until a command line pops up that allows you to type. If you cannot type, then I'm afraid we weren't having the same problem. Please note that my hardware setup is likely a good bit different, since my computer is from 2006; however, here's what saved my butt!
$ sudo apt-get remove --purge xserver-xorg-video-ati xserver-xorg-video-radeon xorg-driver-fglrx
$ sudo apt-get install xserver-xorg-video-ati
$ sudo apt-get install --reinstall libgl1-mesa-glx libgl1-mesa-dri xserver-xorg-core
$ dpkg-reconfigure xserver-xorg
$ sudo apt-get install xserver-xorg-video-mach64
So I suppose this assumes a lot of things:
1) You have internet access at the time of typing those commands
2) You're running Ubuntu
3) You are having a similar problem compared to what I had, relating to poor graphics drivers available for ATI cards, and newer versions of the operating system crashing X because it does not handle the same way anymore, and no better solution has come along.
I think it's clear that I'm no expert. I just had similar troubles to what you're having, and the fact that I'm typing this is indication that I've been able to fix it a few times
. Please let me know how it works out.