Hi, there are a few options.
1. Try a soft reset with the phone plugged into a charger.
2. Try a hard reset using the key combo (Please note this will wipe the phone)
3. Use the Windows Device Recovery Tool to reset the phone (Please note this will wipe the phone and put it back to WP8/8.1)
If you need any further help or have more questions, please join the Windows Central forums, so that you can reply to this thread.