Sorry to double post but I've been doing some digging and found out some info. I couldn't find anything on your specific GMK-20P but I found a lot on similar Hitachi GMK chassis. They are standard res - medium res switchable chassis, which means they can display different resolutions of gameboards. And that is changed by a switch. I have a feeling this is what has happened here, that you flicked the switch and changed the monitor into Standard Res mode while it was on (not advisable!) and the gameboard you are using is a Medium Res board, so the signal was not what it was expecting.
Needless to say this usually fries something, our own grantspain said in another thread I found that the HOT (horizontal output transistor) is usually the culprit.
I think step 1 here would be to confirm thats what the switch does (rub the dust off around the switch and look for any markings, saying Res, 15khz, 24khz etc.)
If this is what's happened then you would need to remove the chassis from the monitor, identify the HOT and replace the part, then hope nothing else was damaged too