Computer Turns on but nothing is displayed

Okay I am stuck on this one. The computer does turn on but nothing will display. The fan spins and the lights on the power button does turn on. Just nothing shows up on the screen. I hooked it to an external monitor and nothing displayed so I thought it could have bad integrated graphics. I ordered and replaced the motherboard, since it is an all in one computer, but I got the same result. I tested my hdmi cable and monitor and they work so I know its not the motherboard or external monitor. I have unhooked everything except ram and cpu fan from the board and that didn't change anything. I removed the ram and it beeped so I know it is doing POST but there isn't any beeps when everything is plugged in. One thing I noticed is that the keyboard and mouse will not light up even after 30 minutes of being plugged in. I know the power cable works because I have to repair basically the exact same computer and I am using the power cable from the first one with it. I don't see a seperate power supply so I believe it is built on the board somehow. So I am not sure where to go because it seems to not be the board and it is going through post but doesn't make a beep. Any ideas?