I had this problem over Christmas - a Pi Zero W v1.1 from 2016 driving the Pi Huts LED Christmas Tree - worked perfectly for about three days, then died. The Pimoroni Zero W layered perspex case was quite warm to the touch, ok hot.
Swapping to another Pi Zero W showed there was nothing wrong with the micro SD card and that Pi is still running flashing LEDs on the tree until tomorrow.
Plugged the dead pi into a 5V PSU [which runs Pi's all the time] with a USB Detector the voltage dropped from 5.2 to 4.3 and it was pulling 1.8A - a running Pi Zero usually only pulls 110mA.
So its very dead - there's a thermal printed label on the back and that was marked where the SOC was cooking.. the label wasn't there when it was in the case, I've started labelling Pi's to keep track of them, at the last count I had about 15, now only 14
