Power but not starting or booting up.

This machine last worked 3 weeks ago. Suddenly, when I press the power button, the fans run, the #1 LED light goes steady green, no chimes sound, screen stays black, no response from keyboard buttons. Disassembled it down to clean tons of dust, and when my brush barely touched it a small round capacitor broke off the logic board. I since replaced the board, and reassembled the computer, but don't know how the temperature sensor plug goes in the 8 pin socket of the Samsung HD502HJ HDD. I've tried all 4 ways to plug it in, and the computer won't boot up after the fans and LED come on. Ideas? I really want to make THIS computer and hard drive work. I don't want to give up on it.