Possibly, by any chance are you using something like a TV for a display. I once had a similar situation where a new video card I installed wouldn't wake up my 47" Sony TV and I vaguely remember having to resort to connecting an actual monitor to the other port (and possibly making it the primary one) before the TV would be given/sent a signal, at which point it would remain on even after redisconnecting the old temporary monitor, until that is, that I turned off computer again, so I had to resort to leaving the old monitor hooked up, not used and sitting on the floor.