My model 50S425 that is a bit over 1 year old, started doing the same thing. Was working fine one minute then shut off a show to put my son to bed, after that the tv would turn on, show the red TCL logo screen then turn back off after about 15 seconds. Then after 10 seconds or so it would turn back on by itself, spend 15 seconds stuck on the red logo screen and turn back off. It would stay stuck on this loop until I pulled the plug. I tried everything I read online. Unplugging made no difference, 1 hr, overnight, still the same red logo loop. Every trick to fix it I read online didn't work.
The reset button on the back wouldn't work either. But I noticed that if I held the reset button down for about 40 seconds it would go to the reset menu. As soon as I let the button go it would go back to the red logo loop. I tried to keep the reset button pressed down the entire time while I went through the reset process but it would come to a point where the tv needed to reboot and it would then act like I let go of the button and go back to the red logo loop. I thought maybe it was a firmware issue and loaded an update on a USB thumbdrive and tried to use that during the reset process. But the TV would not recognize the thumbdrive so it seemed like the USB port was bad. So a bad USB port along with a reset button that only responded when held down the entire time led me to believe this was an issue with the hardware, specifically the main board.
The screen looked fine during the red logo loop and also while the reset button was held down so in my opinion that board was fine, the power stayed on the entire time the reset button was pressed so it seemed to me the power board was fine. I was able to connect to wifi in the reset menu while the button was held down so that board seemed to be working fine too. So I bought a new main board for like $28, only took about 15 minutes to install it. There are some videos online to walkthrough the process but it is pretty simple. Works perfect now. If you tried all the other fixes suggested and none of them work, before you curb the TV, I would try replacing the main board. I had an HD antenna with a booster that used the USB port for power and I have a feeling that fried my board but who knows. At least now the TV works.