OK, I was doing some debugging with Xorg, and thought I had to use the
Magic SysRq key to kill it. But when I had pressed Alt-SysRq-R to
give the keyboard control from Xorg back to the kernel, it turned out
that I not longer needed to do another SysRq because my Xorg magically
worked again… Unfortunately now, everytime I pressed Alt-F4 to close
a window, I found myself on tty4… rather poor. So I needed some way to
tell Xorg to grab the keyboard again, and there it is: Just open an
xterm and execute
sudo kbd_mode -s