Hello, This is often a lockup of the Mainboard in the set, and oftentimes requires replacement of this board.
However it's worth it to try a hard reset, as sometimes the processor in the TV can simply have locked up.
The first thing you would try, is disconnect the set from power, then press and hold the power button on the TV itself (not the remote), for 10 seconds and release. Then plug the set back in, and also check that the power cord is connected tight at the TV as well. Press power once, and see if the set powers up normally. Try this procedure up to 3 times, if the first two do not work.
If the issue is still there (no Panasonic welcome screen, no Input indicator- nothing will display), then the TV needs the Mainboard replaced which is on average a $300 repair on this set and a shop can be located at the link below:
Alternatively if you're a DIY type and looking to repair the set yourself, you can purchase this board at the link below to repair the TV:
Other than that, I'm sorry for the bad news and wish there were a quick fix.
If you need further assistance let me know. Please do not forget to click the 5 STAR RATING, at the top of the page for my efforts. It is my livelihood and you can follow up at any time if needed.