|Just because fans and drives spin up and LEDs flash doesn't mean it's working. It just means those devices are getting voltage. It doesn't mean there's any computin' goin' on.|
Did anything happen to it or did you make any changes between the last time it worked OK and the first time it didn't?
Be sure to unplug the power cord when you're working inside the case. Even when off the power supply provides a small charge to the motherboard and it's possible to damage things when removing and reinstalling them.