iPhone 6, iOS 11 boot loop/restore bar loop
Now that you have confirmed an Error 9 message with iTunes, I can say with certainty that you're NAND IC (that is the chip that has all of the data) is corrupted. This requires removing the NAND and reprogramming it with a specialized tool like this one, reballing the IC and then re-soldering back into place. This will mean that you will lose all of your data so hopefully you have a backup of the data.

This is not a DIY repair, you will need to find a repair shop that does micro-soldering repairs.