When you plug in the molex power cable and turn the PC on, does the drive start spinning. If it is a quiet drive, put your finger on the metal plate (don't touch the circuit board) and try to feel if it is spinning. Is it making any weird noises like a click?
If you can confirm that it is powering up and spinning then shut off the PC and remove the drive. Is your hard drive jumper set to the correct position? The jumpers are next to the molex power connection, a set of 10 pins with a piece of plastic (jumper) that covers two pins. There should be a diagram on the drive itself showing you where to put the jumper when you need to set the drive as a master, slave or CS (cable select). If you are using it as the main disk and it is...