it's panasonic S70 model which I bought brand new about 13 years ago for around $400. It's been working fine for 13 years but since last night, once I turn off the monitor and try to turn it back on, screen remains black even though I see green light next to the power button. I have to plug and unplug the power cord from the back of the monitor few times and then it turns on and have no problem until I turn it off. And then I have the same problem. What could be causing this?