My PC is not recognizing the monitor HELP!

I just replaced the Motherboard and PSU in my Emachines W2888. Everything powers up just fine; peripherals are all recognized (Keyboard, mouse, USB) except the monitor. I tried 3 different working monitors and could not get a picture out of any of them. Could the motherboard be faulty or did I miss a step? Please provide details, thank you all in advance!