A few silly questions ...
Is the computer showing any signs of life?
Is the graphics card one that needs power direct from the PSU, and is it connected?
Have you tried booting it with the disc drive unwired again?
What PSU, Graphics card, and motherboard are you using? - and how many drives are connected?
At this stage my guess is (and it is only a guess) that the power supply is not big enough. I am being premature here, but it might be worth checking.