I've had this problem on my wife's Lumia 950, but not my own. Her phone goes black unexpectedly (twice while taking a picture) and won't turn on. Hard reset with Power+VolumeDown does nothing. Trying to power it from a USB-C cable causes the phone to
vibrate every few seconds, but nothing happens. Windows Device Recovery doesn't work, because the PC can't find the device.
We fixed this three times by removing the battery, letting the phone sit for awhile without it, and then putting the battery back in. When that didn't work this time, I tried putting the battery from my phone into hers and that fixed the problem. I then
put the battery from her phone into mine and caused my phone to reproduce the problem. Per some feedback on this page, I put two small sheets of paper under the battery on the terminal end, to prop it up a bit, and put the battery back in. Now my phone works
with her old battery. So... it seems like it might be a finicky battery connection problem.
She had this problem both with her OEM battery and a replacement I bought her. She also had problems with the battery draining fast, which I suppose could be related.