Hi, just wanted to state that same happened to me twice within the last 3 days, in both cases I wasn’t actively using the phone - just found it to be not responding to anything when I picked it up again and the screen remaining black.
Maybe it could have been related to ongoing app installations? I think that was common for the 2 times in my case.
After all I had to remove the battery and then I could start the phone again.
Besides also random resets happened to me a few times already (e.g. found it asking for SIM pin when I picked it up this morning, which only happens after a reboot)