It's a late 2015 "27 retina iMac. I've had it for at least 2 years. I ran into this problem for the first time while running Lightroom some two weeks ago. Basically, the screen goes black. I can't restart it or force a shutdown. I have to unplug the power chord and plug it in again before it'll start up again, and there are no error notices or anything. I've done a diagnostic – no errors found. I've reset the SMC and NVRAM. Doesn't help. I found a youtube video of a guy describing the same issue and showing how he resolved it (ejecting the RAM then putting it back and restarting), but that didn't help me. I tried it out in safe mode for a few days, and then it seemed to work fine, but it's hard to tell because I can't really work in safe mode. Either way, did a complete reinstall of the system a few days ago and it's been working fine, until today. I installed Photoshop yesterday evening and after a few hours of work today it started happening again. Sometimes it takes a while for it to occur, but when it starts acting up, it will do so often unless I let the computer be for a while. A heating issue? Power? Suggestions?