Why is there a horizontal green bar on the top left of my iPhone X display?

You may have a damaged screen.

First, try a forced restart. If that doesn't work take it in to the Apple store to have it looked at.

• Press and quickly release the volume up button

• Press and quickly release the volume down button

• Press and hold the side button.

• When the Apple logo appears, release the button.

Force restart iPhone - Apple Support (IN)

Make an appointment at the Genius Bar to have it diagnosed.

Genius Bar Reservation and Apple Support Options - Apple

You just described a possible hardware failure that suddenly occurred.

While this is unfortunate, it is not uncommon.

I suggest to Restart your iPhone, which could help resolve this anomaly.

However, if this anomaly continues, then this could possibly mean screen failure.

If that’s the case, then only an Apple Genius can review the cause and provide a complete solution.

Therefore, I recommend service.

Service Checklist:

• iPhone service pricing.
• Check AppleCare Coverage.
• Get your iPhone ready for service.

Click ▶︎ Start a Repair Request.

Need more help?

• Call 1-800-APLCARE.
• Contact Support.
• iPhone Support.
• iPhone User Guide.

Click ▶︎ Check Repair Status. 🔧

Good luck! 👋🏼😉