Monitor "Power Saving Mode" Problem

Hello,

I just bought a computer and only one of my monitor inputs (HDMI, DVI-D, etc.) works at a time. On my old computer, I had a problem similar when my LG Flatron IPS235 monitor would keep saying "Power saving mode". Now it's back, and I figured out that if I have one monitor plugged in, the other one won't have a signal. I don't believe the problem is internal but I am lost on how to fix it. Thanks.