Make sure you have both power cables connected to the motherboard. There is a big one with 24 pins that connects along the front edge of the motherboard and a smaller one with 4 or 8 pins that connects between the CPU and the I/O ports along the back edge. They both need to be connected in order to power up. And as someone else said, make sure the power button wire connector is bridging the correct pins on the front panel pin block of the motherboard. That pin block is usually near the bottom rear corner of the motherboard. Concerning that pin block, there are usually two rows of pins. The two that the power button wire connects to are usually the first two (going from right to left) on the top row. Doesn't make any difference which way you turn the connector. There is not polarity, just a bridge.