MacBook Air Won't Boot * I did an update on my Macbook Retina 13" to the most recent version of Sierra OS X. The computer never gets past the Apple logo (black screen with a full progress bar). *  This is a copy of the problem previously written up in November.  None of the solutions worked nor a few others.
I have maybe the solution, but it requires a USB key and an other computer on macOS. The solution consists to create a bootable USB key.
You can find here a good tutorial: Bootable USB key macOS Sierra - MacWorld
I had this problem too when I installed macOS Sierra (but after I downgraded because my CPU was overheating). I hope it will work for you