I started the Sierra download, WiFi, Mac plugged in. It took a long time, of course. I walked out of the room for a while near the end of the download, and when I returned the Mac was turned off. When I powered up, the status bar would run to about 95%, then the Mac would shut down again. This was repeatable.
But I was able to hold the Option key and boot into Windows. Windows worked just fine (even the brightness keys). I got busy at work, so I used Windows for some weeks before I had a chance to get back to OS X. I tried to repair the OS X partition. It would try 3 times then fail telling me it was not repairable. This was also repeatable. After many chats and phone calls with Apple, I was told that the OS X partition was not recoverable. It was an Apple employee at the store who told me that the file systems between El Capitan and Sierra were not compatible and they way it failed left the boot code unable to recover. Worse yet, in order to repair it I had to erase and rebuild my hard drive blowing away the Windows partition.
Still under warranty, I left my Mac at the local Apple store. They rebuilt the system with Sierra 10.12.6. I assume they erased everything as there was only a single partition on the disk when I got it back. So I then used Boot Camp to partition the disk, install the Windows drivers and install Windows 10. This is where I now have several problems I never had with El Capitan, screen brightness control just one of them. (The other problems yet unresolved are getting rid of that irritating start up noise ((the script file solution worked for a while, then stopped working)) and Windows constantly complaining about a disk not being inserted in the SD card slot.)
Thanks for the attention.
Dave