|If it's a CRT monitor (ie the old bulky type with a tube) it could be one of the colour guns has failed inside it. I could be wrong, but that's what I got when my old monitor died. Obviously your PC is okay if I'm right.|
You need to connect aanother monitor which you know is okay, just to be sure about it. If there's no green screen with a different monitor, then it's def your monitor that's faulty and not the PC.