I checked the owner's manual I could find online and I did not see a master switch on this TV so its not that. You already did a lot of eliminating by trying another cord and another plug. It seems like this is pointing to either a power board or a signal board problem. The power board is more likely to have failed and will need to be checked. If you do set up service tell the technician what it did when it turned off. Some things like a loud pops or horizontal lines give clues as to what else could have gone wrong. The power supply sometimes gets taken out by something else that goes bad in the set, but its very hard to troubleshoot that without a working power supply or visually seeing something damaged.