If your iPhone wont power on, you might want to run through the steps in the following article:
(Note: These steps also work for iPad and iPod Touch)
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The first thing you want to do when troubleshooting your iPhone that wont power on is try force restarting it:
On an iPhone 7 or iPhone 7 Plus: Press and hold both the Sleep/Wake and Volume Down buttons for at least ten seconds, until you see the Apple logo.
On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Sleep/Wake and Home buttons for at least ten seconds, until you see the Apple logo.
If your device still won't turn on or start up:
Second thing to do is allow the phone to charge for about an hour.
After a few minutes, you should see the charging screen, If you don't see the charging screen within an hour, or you see the connect to power screen, check the jack, USB cable, and power adapter. Make sure that everything is plugged in firmly, free of debris, and not damaged. You might want to try a different USB cable or power adapter.
After an hour of charging, try to power it on normally, then attempt another force restart.
If your device turns on but gets stuck during start up:
If you see the Apple logo or a red or blue screen during startup, try these steps:
Connect your device to a computer and open iTunes. If you don't have a computer, try to borrow one, or go to an Apple Store or Apple Authorized Service Provider for help.
While your device is connected, You are going to place the device into recovery mode by forcing it to restart. (Before doing this step, please be sure to check and make sure you have an iCloud or iTunes backup of the affected device) [ Register or Signin to view external links. ]
On an iPhone 7 or iPhone 7 Plus: Press and hold the Sleep/Wake and Volume Down buttons at the same time.
On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold the Sleep/Wake and Home buttons at the same time.
Don't release the buttons when you see the Apple logo. Keep holding until you see the recovery mode screen.
When you get the option to restore or update, choose Update. iTunes will try to reinstall iOS without erasing your data.
iTunes will download the software for your device. If it takes more than 15 minutes, your device will exit recovery mode and you'll need to repeat steps 2 and 3.
If you are still unable power on or restore your iPhone at this point, then you will need a repair for your device. You can feel free to either go to an Apple Retail Store, Apple Authorized Service Provider or call AppleCare 1-800-275-2273 US/Canada for additional repair options. or visit: [ Register or Signin to view external links. ]
AppleCare Support phone number for other regions: [ Register or Signin to view external links. ]
Before you set up / bring your device in for a repair, be sure to:
Backup your device (If possible) / Check for a recent backup.
Turn off find my iPhone on you device: [ Register or Signin to view external links. ] or [ Register or Signin to view external links. ]
If you are going into the Apple Retail Store, in addition:
Know your Apple ID and Password and bring a valid form of ID.
(Making an appointment for an Apple Retail Store is recommended)
Hope this information helps!