Matrox G400Max Dual Head Display Video Card running on XP Pro

Symptom: right-hand monitor dark, left monitor is fine.

Recent History: I rebooted due to a disk situation(fixed now), and
something clobbered the dual head operation along the way.  It had
been working flawlessly.  The cables and monitor are fine (swapping
left-right cables shows the monitors/cables okay).
The dual head mode is set in the Properties, as is the 2560x1024 width.
This happened once a year or more ago, and there was some unobvious
step that i did (as directed by Matrox support, which no longer
supports G400) and it worked fine. I did not write down that trick.
I need debugging help to click the correct parameter to restore the
right hand monitor
thanks