I had the extreme frustration recently of applying some recommended updates in Ubuntu, only to find, on restart, that post login I went to a screen with the desktop background showing, but no icons, no panels, and no right click functionality.
Hmm...
OK, well let's not muck about with that, let's try an upgrade anyway and that may fix it.
Upgrade ran.
Outcome, post login, a black screen - nothing at all.
It appears others have had this issue too, based on using that search criteria. Then, by pure chance, I saw this: http://ubuntuforums.org/showthread.php?t=1643651 and in particular the comment "select "failsafeX", and reconfigure graphics to default"
Somehow this option had passed me by. Plenty of attempts to get to a terminal and waste time from there, but never this.
Of course, it worked immediately.
The downside was I now had a screen with ridiculously large fonts and windows (is this high or low res? Low, I think). Anyway, I needed to upgrade to 11.04, and when I did the magic 1024* 768 setting reappeared - hooray!
And now I've gone one stage further and fully upgraded to 11.10.
All is still good. For now....