I guess it could be the PS, but that's not a very common failure mode where it can power up the motherboard, but not allow it to pass the POST.
Try unplugging everything from the board, USB headers, drive connections, etc. Leave only the monitor connected. See if it can complete the POST then (it obviously won't boot without the drives, but you can still see if it initializes the video).
If you have a spare video card around you could try installing that too. But to me the main factor is still that the keyboard doesn't show that the system is progressing through the booting process.
