Samsung V-Nand SSD 850 EVO 1TB not working
I found the ususal 8GB USB drive was not large enough and only got El Capitan to boot from USB of 16GB or more!

You can see what is happening when you boot the mac using Verbose mode like so:

Right upon hearing the startup chime, press and hold the Command (⌘) – V key combination for Verbose Mode.

As the Mac boots it will spit out lines of code:

You may be able to see whta the problem is from this, or come back and type in teh last few lines fro us to read. Or take a pic with your phone and post that!