This was the first time this happened to my nexus 5 (6.0.1). My phone just went black and frozed on me. I could see the all the WiFi signal, the time and among other at the top of screen. The app I had running was called darker. Its where it dims the screen and gives it a red hue. I tried powering it down but I could do nothing. So I just threw my phone off to the side thinking it just died completely. To my surprise now its working. What cause this? Also, does that mean I need to get a new phone soon? I haven't rooted this phone.