Restart the Chromebook
Force restart (hold Refresh + tap Power, or hold Power for 10–15 seconds)
Unplug all external devices and accessories (HDMI, USB hubs, docks, etc.)
Check for loose connections if using an external display
Reduce display effects: Settings → Accessibility → Manage display size and magnification (adjust if enabled)
Turn off any installed display-related extensions (remove recently added extensions)
Update ChromeOS: Settings → About ChromeOS → Check for updates
Powerwash (backup files first): Settings → Advanced → Reset settings → Powerwash
Boot into Recovery mode and reinstall ChromeOS if updates/reset don’t help
Run hardware diagnostics: Esc + Refresh (hold) + Power (release Power when screen turns on), then follow prompts
If the screen is physically damaged or lines persist after resets, contact the manufacturer or service for screen/connector repair