Desktop icons suddenly shifted to left side

[David M. Karr]

I've been accumulating desktop icons for a while, arranging them in 
precise locations on my desktop.  This has worked fine for quite a 
while, but just this morning something happened at some point (I didn't 
see it happen) that made all the icons "smoosh" over to the left, 
leaving about the right third of the screen uncovered by icons.  Several 
of them were also moved vertically, but to a large extent they were all 
just moved to the left.  Now they are all neatly arranged in rows and 
columns (not the way I wanted them), although oddly the position in the 
very top left corner is empty.

Any ideas what might cause this?