If that didn't work it probably is not a software issue. The other thing to try is a DFU restore. This bypasses iOS entirely and uses a BIOS primitive to reload iOS, rather than have the current iOS kernel manage the restore:
- Install the latest version of iTunes (12.3.1) on your computer if it isn't there already
- Reboot your computer
- Disable any antivirus and firewalls
- Launch iTunes
- Connect the device cable to a USB port on the computer (not to a USB hub, and a port on the back if it is a desktop or tower case), but DO NOT CONNECT TO THE IPHONE YET
- Press and hold the HOME and SLEEP buttons at the same time for exactly 10 seconds, until the screen turns black, then release the SLEEP button but continue holding the HOME button. If an Apple logo appears while holding both buttons you held for too long; start over.
- While still holding the HOME button connect the other end of the cable to the device
- You should see a message in iTunes saying a phone in Recovery mode has been detected (it may take up to a minute for the message to appear). When you see that message release the HOME button and the device should restore. When asked DO NOT restore a backup; set up as a new phone (if you get that far).
