DIAGNOSIS Overheating main board or a corrupted software install.
FIX Unplug the TV. Let it cool for one hour. Ensure all vents are clear of dust and the TV isn't in an enclosed cabinet. Plug it into a different wall outlet, not a power strip. Power it on.
IF THAT DIDN'T WORK Perform a factory reset. Go to Settings > System > Advanced system settings > Factory reset. This wipes all data and reinstalls the OS.
HEADS UP If it restarts during the factory reset, the eMMC storage chip on the main board is failing. That's a board-level repair.