Why doesn’t the LCD turn on when I reboot my Dell Vostro?

I have a Dell Vostro with a strange issue, after few hours on, if I reboot, the LCD doesn't turn on. There is a 2 amber + 7 white blink code (LCD failure) for a while and the OS starts regularly. If I attach an external monitor it works. I already tried to replace the LCD cable and the power supply. They didn't help. Do you have any other idea? Unfortunately it is out of warranty period.

Question by Tobia on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.