Was switching between users on my brand new Late 2015 5k retina iMac and the screen went to black. Nothing but a hard restart would bring the machine back to life. A long press on the back button shut it down, so I know it was still on. Another long press brought up the Apple logo and the start-up chime. That was about 4 hours ago and the black screen hasn't returned...yet. I fear if it's happened once it's just a matter of time before it happens again. An online search for the problem brought up loads of similar experiences for past iMacs, but I haven't been able to find anyone else reporting this for the late 2015 iMacs. I have a Retina 5K, 27-inch, Late 2015; 3.3 GHz Intel Core i5; 16 GB 1867 MHz DDR3 (8GB bought from Crucial); AMD Radeon R9 M395 2048 MB; running El Capitan 10.11.1. I've never seen so many ideas out there about fixing the problem. It runs the gamut from wipe hard drive; reinstall software; get a new video card; unplug everything and plug it in again; try a different keyboard; unplug Time Machine; reset PRAM; remove new RAM, and many many more. I'm not ready to do anything until it happens again, but just wondering if it's happened to anyone else with a late '15 iMac. Thanks! P.S. And of course it happened on day 15 of ownership, so an easy return probably not in the cards.