Black CUE screen, all is working without screen?

Hello,

My brother gave the car to a mechanic to fix the engine problem. But when he picked up the car, the CUE screen was not working. The mechanic did not know what had happened. There is a problem with CUE screen now. I can control all functions, all buttons are working, the digitizer (LCD touchscreen) is working, but I cannot see the screen view. The screen is black all the time. When I put reverse gear, I cannot see rear camera. I checked the second CUE screen. I do not know if it is okay, but it is black too. It is possible that 2 pcs of ECU are broken, but I think that it is a problem with the car. But what is not okay? What is faulty? What should I fix?
I am waiting for your help.
Have a nice day,
Pawel