I have had the phone suddenly go black brick for no apparent reason, after leaving it on the table with a 70% or better charge.
In my case, what worked was holding all three buttons (both volume and power) for about 30 seconds. That seemed to start a reboot cycle, as the Android image flashed up for a fraction of a second, but then stopped.
I did the same again, and again I got the brief Android flash, then nothing.
I then held the power button alone for about 30 seconds, and the phone booted normally.
It showed 76% battery or something at that point.
Good luck.