Followed the directions at http://askubuntu.com/questions/141606/how-to-fix-the-system-is-running-in-low-graphics-mode-error:
- Used Control-Alt-F1 to get an alternate terminal and log in.
- sudo apt-get install --reinstall ubuntu-desktop
- sudo reboot
Update: Well, this didn't work. Next time I booted, I had the same problem...