If no monitor works on the computer, then the graphics card is suspect. Assuming this is a desktop computer, you could open the case and ensure that all cables are connected to the video card but if that doesn't get it done, then your next step is to replace the video card. Of course, you could try reinstalling an updated driver for the graphics card, but it still leads to the graphics card being the culprit.
Hope this helps.
I'm having trouble getting anything from my pc to show up on a monitor. The monitor works, it turns on and says that my computer is connected via hdmi cable, but then the screen turns blank. No sign in screen or anything. Only display is from what the monitor always shows when you turn it on. I'm not sure if I can start it in safe mode since there's not a start screen or anything. I built this computer 2 years ago and it's always worked well, but I'm not so handy when it comes to issues like this. I've tried connecting it to another monitor, and I've tried using a DVI instead of hdmi, but nothing works. Please help!