Steps to Fix:

• On an iPhone 6, the first thing you should do is force a restart by pressing and holding the Home and Side buttons for 10 seconds. The Apple logo should show, suggesting that you've completed the forced restart successfully.
•  If the hard reset fails, connect your phone to a power source and leave it alone for a bit. It could take up to an hour for you to see results, but the phone should display you the 'charging' screen within a few minutes. If you still don't see the charging screen after an hour, Apple recommends using a different USB cable and adapter, as well as inspecting the ports for any debris that might be obstructing the connection.
• Open iTunes after connecting your iPhone to your computer. Hold down the Sleep/Wake and Home buttons at the same time when your phone is linked to iTunes. Keep pressing them until you see the Recovery mode screen, even if the Apple logo displays. You can let go when the Recovery mode screen appears. After that, you'll be asked if you want to upgrade or restore. When you choose update, iTunes will restore your iOS.

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