I've been having the same issues with my MBP 15" late 2011. I worked for over five years without any issues. But after making some firmware update after installing Sierra on it, a few days laters it just stopped working. I couldn't even power it on after that.
So I opened and disconnected the hard drive, the dvd drive and tried to boot.
I was able to get into the maintenance tools and was able to shoot perform an extensive Test.
The service tools were not able to find anything, so I thought "Oh well.. what on earth is going on?"
I reconnected the hard drive, and I was able to boot the machine. First thing I did was of course to backup all my data. After that I made the mistake of shooting the machine down... I haven't been able to boot it ever since.. not even the maintenance tools...
I turns on, apple logo shows, then the screen blinks black and turns grey afterwards and stays there.. I can't even boot from a usb installation drive.
If I let it there like that, it turns off itself after a while...
Lucky me I have a laptop to use from work... but you know.. it is not my machine..