IPhone blacking out

Try recovery mode

If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support

It is in the link I gave -

If you can‘t update or restore your iPhone, iPad, or iPod touch

You can put your iOS device in recovery mode and then restore it with iTunes.

In these situations, you might need to use recovery mode to restore your device:

• iTunes doesn't recognize your device or says it's in recovery mode.
• You see the Apple logo on your screen for several minutes with no progress bar.
• You see the Connect to iTunes screen.

Learn what to do if you see the progress bar on your screen for several minutes.

Put your device in recovery mode and set it up again

1. Connect your device to your computer and open iTunes. If you don't have a computer, borrow one from a friend or go to an Apple Retail Store or Apple Authorized Service Provider for help.
2. While your device is connected, force restart it: Press and hold both the Sleep/Wake and Home button for at least 10 seconds, and don't release when you see the Apple logo. Keep holding until you see the recovery mode screen.

Recovery Mode:

Turn off your iOS device.

Connect the device charging cable to the computer only (not to the device yet).

Open iTunes on your computer.

Press and hold the power button on your device and don’t let go.

Connect the cable to the device while keeping the power button pressed down.

Wait until the computer detects the device and wait for it to restore.

Restore as New first and then restore from backup if desired.

https://support.apple.com/en-us/HT201263