This can happen if the cable is loose (as all the pins don't connect), but since you've tried this with a different video card that's probably not it. The cable could be bad, but it is odd for that to happen out of nowhere.
It may be that the monitor was going bad, and the emulator just gave it that little push over the top?
But, sometimes weird magnetic fields can throw things off (unshielded speakers for example), which will also give you that green colour. Try to degauss the monitor, and also look for a reset button. Lastly, it never hurts to sit things unplugged for a few hours, just to let them discharge. Just a shot in the dark

/bill