Is Power Board the culprit?

@mitchjoskeat "The standby light is also off." pretty much a sure thing that this is your power board. There are commonly a couple of fuses on the board but you will have to remove the back cover etc. Once you do that (unplug your TV first!!!!) then discharge the capacitors before touching anything on your boards. Check the fuses and check the board for any damaged components. It will help if you post some good pictures of your boards with your Question. That way we can see what you see.

Don't just replace the boards. Check those first. It is cheaper and creates a whole lot less waste if you only replace what needs to be replaced. Get a multimeter and let's get to work.

Repair is War on Entropy!!!!!

Adding images to an existing question

The M series from Vizio has a really good picture but the CPU burns very hot and can, after 4-7 years or so, unsolder itself. So the TV starts by being difficult to turn on and it gets worse until it just doesn't turn on at all. A tale tale sign is the white status light in the lower left hand corner will come on and then gradually fade away to nothing. I am not sure that is your problem but it is more common than a bad power supply. If it is, you need to repair the main board or get a used one somewhere with no guarantee that it is not just as old and also ready to crash.