Aha. Bug 546578 led me to bug 555870. I caused the problem to happen, SSH'ed to 
the machine, found out which X display was active:
$ /usr/bin/ck-list-sessions:
Session2:
..snip..
        active = TRUE
        x11-display = ':0'
..snip..

Then did: 
$ export  DISPLAY=:0
and then:
$ sudo xrandr --output VGA1 --gamma 1:1:1 (Your output name may differ use 
xrandr w/o argument to find out)

This restored the display.  For me at least, this problem is related to bug 
555870. 
Others should try the above also to verify if they are affected by the same bug.

-- 
Logout after switch user cause black screen
https://bugs.launchpad.net/bugs/561538
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