Binary package hint: gnome-screensaver
I had my screensaver set to the engine that creates a 3D 4-stroke engine for a screenaver. Lat time it crashed it simply left a black screen. The engine wasn't there and the password box wouldn't appear when I typed or when I moved the mouse. I tried to type the password anyway, but that never unlocked it. I used Ctrl+Alt+Backspace to restart X and just login again, but then I found two copies of engine running in the system monitor. After I killed those of the system was back to running okay.
The screensaver did the same thing to me a few minutes ago. The screen stayed black and I couldn't enter a password. So I did Ctrl+Alt+Backspace again to restart all of X. When I did an error popped up that said X couldn't start. I tried to get to a bash prompt by pushing Ctrl+Alt+F1 but when I got there it ignored any keyboard input I could switch back to Ctrl+Alt+F7 but then it just showed a list of stuff ubuntu had done as it booted, and whatever I had typed below. Finally I just pushed the reset button on the front of the computer and changed my screensaver.
I'll upload the crash file I think goes with this problem, if it isn't let me know and I've got plenty more crash files to offer.
Here's the crash file that seems like it would go with this: