El Capitan frozen boot process
You can troubleshoot El Capitan the same as any other OS X - boot in verbose mode to see a specific log of what is happening so you now the exact area in the boot process when it's failing.

Or you could install 10.11 on to a fresh, new USB drive and get a test that confirms your hardware isn't the problem since a clean install should boot properly.


https://support.apple.com/en-us/HT201573 - How to start up your Mac in single-user or verbose mode
https://support.apple.com/en-us/HT201262 - Try safe mode if your Mac doesn't finish starting up