Restore your device per the following instructions as a NEW device. You can do this using iTunes by connecting your device to your computer using the USB cable that came with your device. See Restore your iOS device to factory settings - Apple Support,
Forget Passcode or Restore Device
How to erase your iOS device & then set it up as a new device or restore it from backups
For help with the above or later models including iPhone 7, 7s, 8 or later, iPad Pro 11" and iPad Pro 12.9" models please refer to If your iPhone, iPad, or iPod touch won't turn on or is frozen.
If the above doesn’t work for you, then:
Follow these steps to place your iOS device into recovery mode: iPhone DFU mode explained, and how to enter DFU mode on your device.
If all fails then make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. If you did not purchase it from Apple then you will need to look to the vendor for service.