Chosen Solution
Hello I am asking for a hint where to look for the problem, after turning on the oled TV, it shows the image for 2-3 seconds, then the image fades from the top. The service claims that the matrix is brocken (the TV is 3 years old). I do not know whether to believe them or they did some trouble checking because the picture shows for a short time, but it is there. My father claims that they connected a device, probably a different T-CON and the symptom was the same. The sound is there.
Jarosław Kurp yes, that is a panel error. It comes from a failed driver board (could be Dc to DC IC as well as many other components). That is the board that connects with the thin ribbon cables to the panel. Those cables are specially bonded and require special equipment to be repaired. Some places can do that repair for you but it is very costly and places that do this repair are scarce. You could try an online search and see if there is anybody within your geographical area that does it. Otherwise you will either need a new panel.
