Have to unplug/re-plug monitor power cable everytime!

Hello,

I have been using ViewSonic LCD monitor VA1616w for a long time and i'm happy with that.
Starting from previous days, I've encounterd a strange problem.
When I pressed the 'PC power button', my monitor don't show anything until I pull the power cable out of my monitor or another end of the cable from the UPS and plug back in.
After that, it works normally until I press 'Monitor power button' off (OR) shutdown and power on the PC again.
Except from this problem, everything is smooth and working very well.
I've tried connecting the power cable directly to power socket, replacing power cable and VGA cable but none of them work. Plz give me some advices :)