I have an older computer that I was doing some maintainence on. After installing a new hard drive, memory, and a graphics card, I tried to turn the computer on and nothing happens. I figured it was the power supply so I put a new one in, and it still doesn't work. I checked the power connections from the power supply to the mother board, they are tight. I checked the connector that connects the power button on the case to the mother board, it is also tight. What are some other possible causes that would lead to the computer not turning on at all?