My laptop wouldn't boot up

Hello,

My Toshiba Satellite L505D-GS6000 laptop has suddenly stopped to boot up remaining with black screen. All lights are on, fans are working. I used this laptop safely for more than five years. I took the laptop to a technician and the technician checked the adapter, battery, hard disk and memory. All are working.
To my surprise, another laptop with the same model of mine has got just the same problem within a week time and brought the laptop to the technician. The technician disassembled the laptops and soldered their graphics cards. Unfortunately, both laptops did not boot up at all.
What kind of a common problem do you guess that would happen on these same products and why? What would you guess the solution will be?
Thank You for your support.