I freaked out this morning when my S7 crashed. While playing a podcast, the audio kept running, but the LED went red. The screen stayed black, the Home button would cause the capacitive touch buttons to light. But otherwise, it was unresponsive. I held down the power button for 20 seconds, nothing happened. The trick is to hold Volume Down + Power buttons and you can get to the maintenance boot screen where you can reboot or power down.
I've been having trouble with mine over the last couple of days. Photos are sluggish when I'm trying to flip through them, programs locking up, bluetooth issues. Typically ends in the black screen and a forced reboot. A couple of times the phone has just re-booted on its own. Did we have a recent update or something?