Hi, I had the same problem after installing a SSD instead of the optical drive. The pins of the vsync-cable were not broken but bent. The first day after the upgrade everything seemed fine, but on the second day the display went black. I opened up the imac and pulled the vsync out of it's slot and left it this way. At the moment everything seems to be all right again. Hope it lasts...
Best regards
Christof
Imac 27 inch, late 2009