Why does "Extend Desktop Across Monitors" not work w/o giving an error message?

I have a Dell XPS 630i w/ ATI Radeon 2600 XT cards (2, joined by Crossfire cables -?)
  1. It's worked properly for months.
  2. I turned it off last night, and now I can only view the 2nd monitor as a clone.
  3. When I check "Extend Windows Desktop to this Monitor" and APPLY - the screen goes black and then it's back as a clone.
  4. The ATI software is Catalyst - same results.

I have rebooted, I don't do gaming, so the Crossfire feature is irrelevant, I have not looked in the BIOS to see if something got changed there (I will as soon as I post this). I am out of ideas.

Thank you for reading.
Maybe that's enough detail. Or maybe not. If you want more, the following is a copy of a query I had on Tom's Hardware, thinking it might be a hardware issue.
Desktop, Quad Core 2.4, XP, both monitors have VGA connectors that go into DVI adapters, have Crossfire cables connected (although I don't know if that matters)
No problems in over a year
1) When I turned it on this AM, one screen did not display anything - "no video signal"
2) I made sure the monitor connection was secure
3) Crawled under desk, wiggled both video connections - the DVI adapter is not screwed in (awkward access) but the VGA plug is screwed into the DVI adapter ----- in retrospect, to remove the "stress" of gravity, I should add a "clip" at the top of the PC box and run the cables through it to relieve the weight of the assembly
4) They both were connected but both had some play - no change at monitors
5) Swap cables at back of video card - now both monitors displayed "no signal"
6) Put cables back in original position, still "no signal"
7) I shut the PC off using the power button, BUT DID NOT PULL THE POWER CORD (idiot)
8) I wiggled the plugs going into the video card more aggressively, seeing if I sensed any play in the PCI card - they did not seem to have play, but I thought I might have heard something, so belatedly I pulled the power cord
9) I removed the cover, made sure both cards (the Crossfire card [?] being the other), put it all back together, and rebooted
10) Dual monitors, but I cannot use them as one desktop - #2 is a clone


My attempts to fix the problem:
* Display Manager / Settings -
*** set up both screens at same resolution
*** checked "extend my windows desktop onto the monitor and Apply (no change, no message)
*** click "Identify" - both show "1"
* Using ATI's Catalyst - Display Properties - Desktop and Display Setup
*** Left Graphic shows 2 monitors, both red
*** Right Graphic shows 4 boxes, 1 is blue, the others are gray - clicking on 2 brings up "Do you want to extend your desktop across both" - YES - no change