We had very similar with my wife's S7 except it died when awake. In our case it stopped dead as she was trying to switch off bluetooth when she heard me driving up as she didn't want the phone to pair with the car. The phone showed absolutely no signs of life at all. There was no indication of charging. It didn't respond to press and hold power or power and vol-down or any of the other combinations of home key, vol-up, vol-down, power, plugging in cable etc that you read on the internet. It was dead as a post. She couldn't go long without the phone so we took it to an expert the next day who instantly diagnosed an internal hardware fault that he was unable to repair. It would need a mainboard repair or replace which would cost more than the phone was worth. So we replaced it (with a Samsung A71) and swapped over the SIM and SD and most of the data restored itself from cloud or SD.
The story doesn't end there... After a couple of days I tried charging again and the charging LED was lit, and charging graphics came up. I let it charge right up and it powered up OK but after a few minutes of frantically trying to upload the data that we lost, it died again. The back of the phone was warmer than it should be. I figured that whatever internal fault the mainboard had was causing it to heat up and discharge the battery too fast. So for my next attempt I put it in the freezer overnight. On next re-start, I put it into flight mode and only turned on wifi - needed to upload data. This time we recovered a few photos before it died again. After a few more attempts, I found that I could get all the lost photos off it by freezing it, charging it from frozen to about 30% then powering it up while still cold. I switched off all wireless functions then sat the back of the phone in ice water with a USB drive in the USB port and copied all the files we wanted directly to the USB drive. This was much faster than wifi and clearly wasn't taxing whatever part of the hardware was faulting and causing it to die. I think the ice water helped to keep it cool for longer operation but if I needed to do it again, I'd fashion a waterproof shield around the top half of the phone with aluminium foil and blu-tac so that more of the back of the phone could be submerged. As it was, I needed the USB port above water and if I let the top of the phone dip under the water, the wave of water would activate the swipe down from the top of the screen.
Our fall-back position would have been to send the phone away for a chip-out data recovery for which we were quoted $180NZD with no guarantee of success. This wasn't worth it for the data involved but we were happy to be able to recover what we wanted ourselves.
Good luck if you have the same issue - and try the same fix. Our new phone is setup much better for doing regular backups to the cloud.
. So I'm going to make one suggestion anyway, just in case: have you tried pressing and holding the power button for 10-20 seconds or so? This is the "forced reboot" procedure for most devices, and if you just have a system crash of some sort this should get it going again.
