Typically, black screens are caused by hardware problems with the iPhone. Due to this, there is usually no quick fix solution. However, software crashes can cause the device display to freeze and turn black and a hard reset could possibly resolve this issue.
To perform a hard reset, press and hold the power button (sleep / wake button) and the Home button (round button at the bottom of the face side of the phone) at the same time until the Apple logo appears.
- If you are using an iPhone 7 or greater, hold the volume down and power buttons simultaneously until the Apple logo appears.