I went into freak out mode when my 4 year old, rarely used 42 inch Panasonic plasma did the same thing. I did the whole "depress the power button for 10 seconds" "unplugged it for long periods of time" and "plug it into a different circuit" Boom! Still the same problem. I unplugged it one last time and I got up and went to Best Buy to possibly buy a new tv. Once there after seeing the super low prices of better technology 42 inch tv's I thought maybe it was time to purchase something new anyway. After about an hour in Best Buy, I decided not to buy anything new and there is no way my 4 year old Panasonic was broken and needed a repair shop. When I got back home after being away about 3 hours, I plugged it back in with no true expectation that it would work. Boom! It's still not working!. I did the "depress the power button" again, still nothing. So I gave up and unhooked all the components (Comcast Cable Box, Sony Google TV player and a Samsung Blu Ray player, all connected via HDMI) and carried it to an unused room in my home and made room for another TV. Once in the other room I plugged it in and Boom! It worked. So now I'm really puzzled.
To possibly narrow down the problem, because it was too heavy to carry back and forth, I hooked it up to an extension cord and plugged it back into the same circuit that it was initially plugged into and it was still working. I re-hooked the Sony Google TV player to it and it still worked. I then took the TV back to its original spot and reconnected all of the components to it and it worked fine.
The only thing that I can think was the problem was the long period of time that it was disconnected and the fact that after I did the reset, I unplugged it again and re - plugged it regardless to where it was plugged it worked. Because now it's in the original spot and working fine.
The only other thing I can think of is it was originally within close proximity (not on top) of a floor heat register. I have since moved it about 6 more inches away from it.
I guess you have to be patient and try a combination of simple things to remedy the problem before you start removing the back of the TV and replacing boards, fuses, circuits (especially if you have no experience). And as the BB salesperson placing it on craigslist to sale for parts or even placing it on the curb for trash. It could be something very simple. But a new 40 inch Samsung Smart TV for $300 less than I paid for the Panasonic sure sounded good.