Have you charged your iPhone. If not, charge it to 100. You can try to force restart it, too.
On iPhone 7/7 Plus
Press and hold the side button and the low volume button for at least 10 seconds until you see the Apple logo.Restore Your iPhone to Factory Settings.
Step 1 Launch iTunes and connect your device to the computer with a USB cable. Click the iPhone icon in iTunes interface.
Step 2 Tap on the Restore iPhone... option.
Step 3 You'll see a message popping up asking if you want to Backup your iPhone, we suggest you to confirm it since you're going to wipe your device.
Step 4 Click Restore button if you get ready to restore your iPhone and then wait for a while until it restarts.
Step 5 Now your iPhone will appear as a totally new one as the first time you saw it. You can restore it from the backup to get your data back.
But this method is the best: use
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