When starting the car the screen is completely black?

Yep, the video driver inside the unit or the screen itself sounds like it has gone bad. This is a common problem because of the heat and cold vehicles go through in everyday operations. Can I ask if the rear headrest display screens are working? If not, it could be the AV control unit, we should do a CAN scan to see what codes are stored. You can get a CAN scanner (Controller Area Network) which will work on most cars from Amazon.

Here is a video to show you how:

https://youtu.be/u-4syLc-ifQ

Here are the six fuses that run the system so you can double check for a blown fuse, this guide can show you how:

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Here is how to remove the screen unit so you can send it in for testing. Search Google or Ebay to find the best place for the testing. Check out the images (below). Please upload pictures or videos of the problem so we can see what's going on.