Won’t Boot Up

I have a gateway 310. It will not boot up. I have changed the motherboard which is a thrasher motherboard 4000812. I changed the fan and heatsink because the fan stopped working. I changed the power controller. After all of this, the computer still will not boot up. The fan is not turning at all on the power supply. When I plug in the computer and press the power button the motherboard flashes a green light but it still does not boot up. I unplug the computer and the light stays green then fade away. I turned of the computer and unplugged the connector that runs from the power supply to the motherboard and the flashing stops but as soon as I press the power button the flashing starts again. Can anyone help me?