The touchscreen is not working

Hello! I was using my Sony when the screen turned completely black. I connected the notebook to the TV and could see the images on it. The touchscreen worked perfectly. I took the notebook to an unofficial service, and when he returned my notebook I could see the images, but the touchscreen didn't work. He says he simply touched the contrast of the keyboard, but I dont believe him. I rebooted the O.S., but still failed to recognize the touch system, so I guess it's a hardware problem. Anyone can suggest a solution?