Did you check the input? Some monitors are set to DVI as their default input (or vice versa) and will not display anything if you plug in a VGA cable (or vice versa). Just a setting you need to change on the monitor.
Or you can try moving your graphics card around. Could be a bad slot on the main board.
And it's probably not caused by a virus: Viruses are typically loaded after your operating system is loaded, so you should still be able to see the POST.
Human history is the story of complaisance. While disaster is fresh in our memory, we take precautions. But as the memory of disaster recedes, we start to take risks.