Fortunately, a one-shot solution is just to restart the X Server. Go to a terminal, eg, by keying
Ctrl-Alt-F7to go to a
ttyafter reaching the log-in screen at startup. Then do
sudo restart gdm.
This requires that you can use
sudoof course. At least with Ubuntu, that's the same as doing
sudo /etc/init.d/gdm restart.
On startup, Ubuntu does detect the problem and offers a choice to restart the X server, but that somehow doesn't work. Doing this manually at a terminal does though. Weird.