When I use on board graphics for my computer it works fine, but as soon as i plug in the graphics card there is no display at all on my monitor. I'm new to this so I plug in an HDMI cable in to the graphics card and nothing happens. There is a cable that comes with the graphics card that i believe is a VGA cable but it is ridiculously short. The display does not show either if I plug in my HDMI cable in to the MOBO, but as soon as the graphics card is disconnected it works fine.
That normal for older system to switch to gpu when you install a new gpu.
What the make of your pc and the wattage of your power suppy and the model of the gpu. Your power supply coud be under sized for the new gpu or your not plugging in both power plugs into the gpu.