How to Fix Grey Horizontal Lines on iPhone Screen
IPhone software and hardware issues can cause your iPhone to display vertical grey lines intermittently or permanently. The first way to troubleshoot this problem is to eliminate software factors. If the lines are present because of a hardware issue, then you will need to get it repaired at the Apple Genius Bar or by an authorized Apple repair technician.
1 Software Fixes
If the lines appear only in a certain app, then double tap your "Home" button and swipe up across the app to quit out of the process. If the issue persists, do a hard reset by holding the "Home" and Sleep/Wake buttons simultaneously until the Apple logo shows. Finally, you can back up and restore your iPhone via iTunes. This will reset your iPhone to factory settings and erase buggy firmware glitches.
2 Hardware Issues
If software troubleshooting does not work, then your iPhone might be affected by a display issue, such as a loose connection or physical damage. Book an appointment at your local Apple Store Genius Bar (link in Resources) or visit an authorized Apple technician to have your display repaired.