My daughters I phone screen is black. Her I phone watch is telling her a phone call is coming in so the phone isn’t dead and she can talk on it but no screen is visible

1. Restart the device. That's it.
2. Still not solved --> Force restart iPhone - Apple Support
3. Follow the Apple article to resolve the issue --> If your iPhone, iPad, or iPod touch won't turn on or is frozen ...
4. If not resolved; please follow the steps in the following link for support with this issue: If your iOS device restarts or displays the Apple logo or a spinning gear unexpectedly

• You still see a black screen after charging, force restarting, following the steps in this article.
• You see a black screen, but you can hear alerts, sounds, and vibrations.
• The screen remains black, but the device chirps when you unmute it and connect it to power.
• Your screen is on, but it doesn't respond when you tap, slide, or try other gestures. 
• You can't force your device to restart because a button is broken or stuck.
• Your device is stuck on the Apple logo, displays a solid color, or isn't recognized in iTunes.

Contact Apple Support to set up service if you have any of the above issues: