which cable is causing the problem?

Hey @iforani, here are some things you can try:

Check The Connections

Before you can confirm something is broken (such as the LCD) check to make sure the connection of the flex cables and the logic board aren't dusty. If it is, clean it off with 99% isopropyl alcohol and a brush. Reconnect the flex cables is make sure the flex ribbon is not bent more then 90 degrees. If it is bent, it needs replacing.

More Possibilities

I found this guide here which lists more things you can try, to fix this issue.

Replacing the Display

If the issue still persists after checking the connections, you'll need to replace the display. Follow this tutorial on replacing it.

Still Not Working?

If you've tried everything above and the issues still persist, you might have a corrupted OS and restoring the device is the way to fix this.

-Let me know how it goes.