If your iPhone suddenly goes black and won't turn on, don't panic. First, try a forced restart. Press and hold the volume up button, then the volume down button, and finally hold down the side button until the Apple logo appears. If that doesn't work, connect your phone to a power source and let it charge for a while. It could be a drained battery. If it still doesn't turn on, you may need to take it to an Apple Store or an authorized service provider for diagnosis and repair. ????
