If your monitor is in sleep mode (yellow light) that means there is no video signal from the computer. If you restart you don't see anything at all then it could be the video card issue. If you changed to the refresh rate to a mode the monitor and it can’t control over it so you have damaged your monitor you can have check by testing your machine with other monitor other than your friend machine or remove the video card and try it on your system integrated display if you can see the image it means you have to refresh your Video card and reset it or try to change the PCI slots and check it if it does not display on monitor then the issue was relating to your Video card.