I’ve had several of these now, refusing to come out of standby. At switch on it’s possible to see the screen light for a split second, sometimes doing this several times before going back to standby. You may need a darkened room to see this. The reason it does this is failure of some of the backlight LED’s, at switch on the LED driver senses a problem with the current being drawn by the LED’s and shuts the power supply down, thus saving any further damage.
This is not one of Panasonic’s best or most expensive TV’s, in fact it’s one of their cheaper range, but still worth spending the £120 or thereabouts it will cost to be repaired by a qualified engineer.
I often get asked advice about DIY repairs on these screens, the best advice I can give is to not even try, there are too many things that can go wrong if you’re not 100% sure of what you’re doing. I’ve had to console several people who damaged the tabs on the screen, very easily done, yet impossible to put right, so please take your TV to an expert!