This helped me (on my Sony Xperia Z3 D6603 version, but it should work for other models) after i tried everything else, it happens when you let your phone drain completely and you try to turn it on again - it leads to: it won't turn on, it won't charge (no led of any color whatsoever) PC operating system cannot detect it (or it detects it as unknown device) and PC companion software won't detect it and it doesn't react when you plug it in with 5 sec vol down hold. Button combinations mentioned above wont work at all, phone will not vibrate.
This is not my solution, i read it on Xperia forums but it was burried among useless posts of people replacing their phone as a solution (which i would consider as spam).
The 2-minute restart procedure.
Detach the SIM card slot cover.
Using the tip of a pen or similar object, press and hold down the small OFF button (generally indicated by the text "OFF") for 120 seconds (ignoring any vibrations), and then release the OFF button.
Charge the device until a charging icon appears on the screen.
Turn on your Xperia™ device.
Also, important notice: Before i held OFF button for about 120 seconds, first time i held it for around 60 seconds, the result was that when i plugged it back to USB port, LED turn on and it was red. I tried button combinations again and none of them worked, so i decided to leave the phone for around 10 minutes to charge and then WHILE IT WAS STILL CONNECTED TO CHARGER CONNECTED TO WALL SOCKET i held yellow OFF button for 120 seconds. It started vibrating 3 times per each session after ~65 seconds and then it continued in regular intervals until i stopped holding it at around 120 second mark. After that, phone turned on, red battery icon that signals that the battery depleted appeared. After that, it took couple of minutes for the phone to reset itself and it started to charge showing 1% charged. I left it like this for now since i'm still under procedure of turning it back on again. I will update when i turn it on completely.
Result: Phone was at 11% charging mark when i pressed Power button, phone turned on and it is working normal. So this solution works.