A black screen is usually caused by a hardware problem with your iPhone, so there usually isn’t a quick fix. That being said, a software crash can cause your iPhone display to freeze and turn black, so let’s try a hard reset to see if that’s what’s going on.
To do a hard reset, press and hold the power button (also known as the Sleep / Wake button) and the Home button (the circular button below the display) together for at least 10 seconds.
On the iPhone 7 or 7 Plus, you perform a hard reset by pressing and holding the volume down button and the power button at the same time until the you see the Apple logo appear on the screen.
And if you have an iPhone 8 or newer, perform a hard reset by pressing the volume up button, then pressing the volume down button, and then pressing and holding the power button (iPhone 8) or the side button (iPhone X or newer) until that Apple logo appears.