Well, this might be a long shot but still... I recently changed the case of my PC to something a little more snazzy. When I hooked everything up and turned it on, there was zero activity. No lights, no fan noise, no nothing. Figuring I had either fried the main board or the power supply, I exchanged the latter for the one from my old case. Nothing. I just could not believe that carefully removing the mother board, taking care to follow proper procedures to eliminate static, could have damaged it.
Then I noticed something funny. The little block of pins where all the connectors to main switch, reset switch, LEDs and so forth went, was missing a pin. One of the tiny connectors had just a single pin connected to it. I looked in the manual, yes, orientation was correct, so it must have broken off. Bummer. But no, that wasn't the solution either: those pins are strong, and require a lot of force even to bend them. Then I decided to see what would happen if I turned the picture in the manual upside down, and connected everything that way. Lo and behold, the empty space was no longer occupied, and when I pressed the power button, everything started bleeping, turning, humming and buzzing. First time I ever followed instructions to find out it didn't work.
The moral of this story: are all of the connectors really properly attached, in the correct orientation, to the right socket?