Applying pressure at that spot is perhaps the answer. It confirms my own experience and fix. My WH1000 XM3's did the same but remained off. They charged correctly, showing a red LED, but simply failed to turn on. However when i plugged in the supplied 3.5mm audio cable directly from a sound source that worked fine. Just could not turn on and the powered electronics is what has failed. Pressing the on/off button produced a faint blue flash on the LED and that was it. I tried all the soft resets but nothing so I then opened up both head pieces and even went as far as ordering a replacement the battery etc, but no change. I later observed online where a few people had some success by simply tapping or knocking the "left" headpiece. I am not in favour in random knocks like this to sensitive electronics but it got me thinking that this is perhaps a physical connection issue somewhere ?. So I opened up the left headpiece, it's relatively easy and you can get video or picture footage of this procedure online. But I then very carefully slightly moved and reset the multiple wires and retaped a couple of black pieces of tape to keep everything down flat. To be honest nothing is really changed. A very suspect connection in my view is that plastic orange flat ribbon cable at one end. This plastic flat cable connection comes from the power and ambient buttons in another concealed section. Whilst re-taping down the wires and reseating the flat cable i was constantly pressing the on/off button and suddenly it just turned on with the solid blue flashing LED. I was fortunate to get it working but after putting the left head piece back together it then immediately failed again. So I then re-opened it and reseated the cabling moreso than the first attempt and this time it seems to remain working. I cannot determine precisely what connection fails but my point is this, some connection within this left headset piece is rather sensitive. the pressure point mentioned in the earlier post is the likely problem fix.
In Summary, The WH1000 XM3 model is a supreme headphone and when it works is awesome but i feel that Sony has under developed the wiring. I cannot discount the fact that a fall or electronic heat may have triggered this fault with my set, but reliability is clearly a problem here.