My iPhone (not even a year old, Xs) died all of a sudden today, even though it was about 75% charged.
I finally revived it by trying the Device Firmware Update button sequence several times:
Press & release Volume Up button;
Press & release Volume Down button;
Press and hold the Sleep/Wake button ..
.. and then the Apple logo came on and it came back to life.
Who knows what constellation of firmware, software and hardware settings was to blame for the system ‘s black-out .. but from now on, I’m going to close all open screens and turn the phone completely off & on again, at least once a week.