My computer had been asleep most of the day, and I just now woke it, everything went on fine, then about 30secs later the screen changed to a solid, dark green. I tried turning the monitor on and off, unplugging then plugging back in, tried a different HDMI cable and a different monitor, no change. My PC is about 4-5 years old, upgraded the video card earlier this year but no problems until now, even with long gaming sessions. Any advice on what could have been the cause? It's only happened the one time, but I'm worried it might happen again.