Question MoBo turning on without power button, 100% fans and no boot. Dead?

[KippRay]

Stripped my dell precision down to change the fan and now the desktop wont boot and powers on as soon as power cable is inserted.
Every part has been cross tested in an Optiplex (Bar mobo) and is working fine. The other working cpu doesnt work on the mobo either.
This happens even if only the 24 pin mobo cable is connected. Nothing else.

Any help would be appreciated.

 

[Nine Layer Nige]

If you took the mobo out, is the mobo now shorting out on the case somewhere (jammed loose screw, etc) ?