I booted into recovery mode from the Grub menu, which includes an option to fix X problems - which resolved nothing. Googling uncovered the following command:
sudo dpkg-reconfigure xserver-xorgThis should have taken me through some options for resetting my graphics settings, but instead took me through options for my keyboard, which again resolved nothing.
One of the great things about the Ubuntu community is that there is never any shortage of help when things go wrong. However, one of the difficulties is filtering out obsolete advice, as there can be major differences from one release to the next. In this case, it seems that in more recent versions on Ubuntu, the graphics settings have been moved out of xserver-xorg into the driver utilities themselves. For my ATI card, the magic command to reset the graphics was:
sudo aticonfig -f --initial(Thanks to: http://www.phoronix.com/forums/showthread.php?t=14015)
I haven't tried this, but my guess from the man pages (http://linux.die.net/man/1/nvidia-xconfig) is that the equivalent for Nvidia cards would be to:
sudo mv /etc/X11/XF86Config /etc/X11/XF86Config.old...which should cause it to re-initialize the X configuration. Use at your own risk..!