I tried to install new video drivers manually, which failed entirely. I only was able to use a GUI again after doing
sudo apt-get install --reinstall ubuntu-desktop
sudo apt-get intsall fglrx
And now I've got my desktop and everything back. But, I can no longer use Minecraft or Steam, they both crash, complaining about an X error:
Bad Request (invalid request code or no such operation)
What should I do to fix it?