Restart the phone
Remove the phone case
Clean the screen gently with a microfiber cloth
Check for screen cracks or physical damage
Take a screenshot to see if the lines appear in the image
Boot the phone into Safe Mode
Update the phone software
Uninstall recently installed apps
Adjust display settings if available
Reset display settings to default
Back up data
Perform a factory reset
Check for loose internal connections if the phone was dropped
Replace the screen if the display is damaged
Contact the manufacturer or a repair center