The easiest way to determine which part has gone bad; I would suggest you to plug in an external monitor to the notebook. Once it's connected, press and hold down the FN and press F8, this should display the screen on the external monitor. If the external monitor shows the picture fine, then the problem is with the LCD.
If the external monitor has the same problem then it could be a problem with video drivers or with the video card itself.
You can even follow the below mentioned steps in order to determine the issue:
1.Test the LCD screen in the System Setup (BIOS). In order to get to BIOS, restart your computer, when the Dell logo appears, tap <F2> until the message “Entering Setup” appears.
2.Run the Laptop Built-In Self-Test (BIST).To run the Built-In Self-Test follow the below mentioned steps:
i.Turn the computer off.
ii.Press and hold the “D” key while pressing the power button. This will start the BIST.
iii.The screen will display multiple flashing colors. Look for lines, distortion, or any other video issues you have observed.
iv.The laptop will restart when the test is finished.
Please reply with the results and any further questions.
Thanks & Regards