Most likley you have bent a pin in the processor socket so that one of the x1 DINNS is not being seen.
Populate just the blue slots and see if you can boot to BIOS set-up.
It should show which bank is not being seen.
I had the same problem, and after some research I found out that the s2600CP4 board does not support 1600MHz UDIMMS, only up to 1333MHz UDIMMS. If you have registered dimms, it should not be a problem...
Hope this helps