ASUS LCD monitor won't display in VGA or HDMI

Hello,

I have a ASUS monitor that will display VGA on a laptop, but HDMI will not display. When I plug it into my PC tower, it will not display in either VGA or HDMI. I have tried plugging in, then powering on, and powering on, then plugging in. Anyone have any thoughts? The Laptop is a Gateway, the tower is homemade, but I do know that an older monitor has displayed on it.