You might need to force restart it. When you get no
If you have iphone 8 or X and later, try this. Press volume up and release it. Press volume down and release it. Then press and hold the side button and you should see the apple logo appear on the screen.
If you have iphone 7 you can press and hold side and volume down buttons together at the same time for 10 seconds or more until the apple logo appears.
If you have iphone 6 or ipod touch or any ipads, press and hold both the home and top / side button for 10 seconds or more until you see the apple logo appear.
This black screen issue can happen if your iphone crashes for any reason, rare, but it can happen.