I assume it will shut off if you unplug it from the AC wall outlet. Some TVs do come on when you plug them back into the wall so try turning it off after you unplug it. Maybe leave if unplugged for 5 minutes.
Try doing a factory reset in the TV menu after that.
If nothing works you may need to replace a board in the TV.
Check with
www.shopjimmy.com
They sell replacement boards and are very helpful.