We are a repair shop having the same problem today during a a screen replacement job for a Inspiron 15' 7559. Device came in not powering on, I thought it was battery flat so I just proceeded without testing to put the new screen on. After putting the new one on it I found it was still not powering on, it stuck at a black screen and LED blinking 2 amber then 7 white as described in the heading.  Therefore I put back the old screen but it was the same black screen. After some play around and switching the old & new screens, pull and putting the connector back and secure it with tape etc etc. In one of the million times I made it to boot into Windows normally, everything went fine so I put everything back with the screen on and shut down the machine. I tried to run a last test before calling the customer in, found it came back stuck at black screen again. I touched nothing at all and the machine shut down like normal, no flickering no nothing.  I followed instruction found on some other threads here to long press D then power on, it ran on a self test. So I did with the old screen on, nothing happens. Then with new screen on, I could see the screen have slight color changes this time but without any back light at all. Putting the old screen back on again, this time backlight lit up but screen was badly cracked so I couldn't see anything, and after just a hard power off, it came back to the black screen again. SOOOOOOO frustrated! After looking up the fault code, the LED blink shall point to LCD problem. However the tests result switching between the old & new screens was kinda on & off, there was no solid conclusion of whats wrong also I couldn't make another successful boot into Windows. In one of the long press D self tests the new screen has lit up and it could run thru the entire self test with different solid color screens but it just wont boot into Windows, and I could not make another self test run with backlight on. I started playing around with the LCD cable, cleaned the socket and connector with alcohol and tried again, same black screen. Then I have gently twisted the cable and putting the old screen on. It booted again! So this time I have hooked up an external monitor and did the BIOS update immediately. After the BIOS update I put on the new screen very carefully and now it boots every time now. Customer has picked up the machine and both of us are happy now ;) Conclusion: I really dont know if it was the cable or BIOS update did the magic for me but I will suggest to first update the BIOS first if you are experiencing this during a LCD repair. If nothing has been replaced and it happen suddenly. The LCD cable will lead to this therefore try pulling the bezel around the LCD, unscrew the 4 screws and take the LCD out. Carefully lift the tape and use the little 'handle' to gently slide the connector out of the socket in opposite direction of the tape, then putting it back carefully. Do not touch the connector, do not bent or pull the cable by force, it is not that fragile but it only take 1 hit to kill it if you do it improperly.   Good luck! Answer from phonefixhk on dell.com