If you have your phone repaired by a third party repair facility, your iPhone may become bricked from the so called 'Error 53.' It appears that the secure enclave gets disabled during a repair if the repairman doesn't know how to fix it.
Apple has released a new update to iOS that will fix Error 53 and revive the phones. Touch ID won't work until Apple resets it, but at least you will have your iPhone back. Read more here - https://support.apple.com/en-us/HT205628