If your iPhone gets stuck on a black screen for the first time and then won't turn on, it might be due to small system issues that can be fixed with a soft reset. However, because your device is not responding to touchscreen input commands in its present condition, you will need to use the force restart technique to restart it. It's still a soft reset, but this time it's done with the hardware keys. There will be no data loss because the method will not alter data. Here's how you do it:

1. Quickly press and release the Volume Up button.
2. Quickly press and release the Volume Down button.
3. Hold down the Side/Power button until you see the Apple logo.