Just installed 12.10 on an HP Elite 3800 Desktop with ATI Video, all worked GREAT ... (openSUSE didn't even want to install)..anyway, did the updates then, and now in only boots up in a blank screen, first a purple screen, then it becomes paper like beige background, try to go to a terminal, (CTRL-F2 etc.), results in black screen, also can get back to GUI...
I basically can do anything with the pc, nothing to type etc.
Try to open a terminal with ctrl + alt + F1. Then you can mess around with the unity greeter, ubuntu-desktop and your graphic drivers.
Some of the answers here might help in your case: How to fix "The system is running in low-graphics mode" error?
You may try:
sudo apt-get install --reinstall ubuntu-desktop or If really stuck, backup your files and reinstall. It may help to locate the update that causes the problem. (I empathize: I am myself in this situation where I am almost certain that a update/reboot would crash my computer. My model is Samsung series 9)
In my case, the likely update is the unity-greeter update tough I am not sure. My symptom is that I am unable to login (even with correct password, the login screen reapears). If I boot in failsafe, I get "System running in low graphics mode".
Procedure for backing up your files: (in the terminal that you access with ctrl+alt+F1)
$ # Insert flash drive
$ ls /dev/sd*
$ sudo mkdir /mount
$ sudo mount /dev/sdb1 # maybe sdc1, sdd1...
$ sudo cp -R /home/yourname/ /mount/
$ sudo umount /dev/scb1
$ # remove flash drive and check your files on another computer
$ # reinstall
