The cause of green lines on an iPhone screen can be attributed to a hardware malfunction. Depending on the severity of the issue, there may be a variety of ways to fix the problem.
First, try restarting your iPhone. To do this, press and hold the power button at the top of the phone until you see the Apple logo appear. If restarting your phone doesn't resolve the issue, you may need to visit an authorized Apple repair center who can use specialized tracking technology to identify and repair the issue.
Another option is to try resetting the phone's settings. You can easily reset the settings by going to the ‘Settings’ app and tapping 'General' followed by 'Reset'. Then select ‘Erase All Contents and Settings’. Once you have done so, the device should come back up as a new device.
If none of these solutions work, it's possible that the issue is caused by a faulty LCD screen, which will require replacing. You will need a professional to replace the LCD screen for you. This should fix the issue and restore your iPhone screen to its original condition.