Computer turning on but showing nothing

I have a Mid-Sized Tower Windows PC. It's been working fine up until now, but it has been acting a bit wonky lately. It's 5 years old.

Today, I turned it on and... Nothing. The light turns on, I can hear the fans whirr to life and such... But nothing shows up on screen. My 2 monitors say "No Signal". It was working just fine yesterday and now I don't know what's happening.

I did try to troubleshoot every way I could think of. I turned it off and on. I unplugged and replugged the monitor cable. I even thought that may the GPU might have crapped out and replaced it with a better one I planned to put in the new computer I'm building. Nothing.

I'm at a loss here. Anyone have any advice?