DIAGNOSIS This is the classic 'backlight failure' or 'LED driver' issue on these models. The panel is getting a signal and the TV is technically on, but the lights behind the screen aren't illuminating.
FIX Power cycle it completely. Unplug the TV from the wall, hold down the physical power button on the TV itself (if it has one) for 30 seconds, then plug it back in and try turning it on. If that doesn't work, you're likely looking at a hardware repair. For a TV under warranty, contact TCL. If not, a repair shop can often replace the LED strips or the power supply/backlight driver board—it's a known point of failure on this series.
IF THAT DIDN'T WORK Try a factory reset. With the TV on (even with a black screen), press and hold the physical power button down for about 10-15 seconds until you hear it click off and then back on. This sometimes forces a reboot that a remote power cycle doesn't.
HEADS UP If you see a very, very dim image you can only make out with a flashlight, that 100% confirms the backlights are dead. Don't waste time on more software fixes.