It seems like the problem lies in the LCD itself but here are some troubleshooting steps that may help.
- First, connect an external monitor to the laptop then press the Windows key + P at the same time then select Duplicate display and see if the same problem will persist on the external display.
- If you'll still see the horizontal lines on the external display that could only mean the the problem is the GPU itself.
- Do a clean install of the graphics card driver that may solve the issue.
- Go to Device Manager and uninstall the graphics card driver.
- Next is to open Programs and Features then uninstall anything related to the graphics card.
-Once done, download and install the latest driver from the manufacturer site of the GPU.
- Reboot the laptop once the latest driver has been installed.
- After the reboot and if the same problem will persist I would suggest contacting Dell support for a repair if it's still under the factory warranty.
- But if the line will not appear on the external display, try to open the laptop and check the ribbon cable that connects the LCD to the motherboard. If it will still not work after reseating the ribbon cable, last option would be to replace the LCD already.