Why did screen go white?

today I was working on my Samsung NP300E5E Laptop, and the screen suddenly turned white, I tried to reboot, it was still white, I couldn't see bios or anything, except it showed the Samsung logo (the one that normally appears for a few seconds whenever you turn the device on) and then it went white again.

one year ago, the screen has been damaged before leaving behind a black strip on the corner, but that was not much of a problem to me, the black strip/spot has been there for over a year, and I lived with it. but now that the whole screen turned white, I totally can't see anything.

I tried to plug in an external monitor via HDMI, and it worked, and I was able to log in the windows and everything seemed normal. non the less the main problem was not fixed. I read many forums and they all suggest that the video cable or screen might be damaged, but I ruled these out since I was able to see the Samsung logo at the beginning, and I tried to press F2 (when the logo screen appeared), I could see the bios setup screen for a second, but then it went white again.

I don't think it's cable problem, video card problem or screen problem, since external monitors works and the screen does actually work at beginning (at least for a few seconds). and by the way, the screen only turns on temporally on the logo, and only the logo, so it's not that it got fixed because I moved or twisted the cable a little. it was totally independent of the cable. the logo keeps appearing every time I turn the laptop on.

'''What could the problem be?

can I do anything else to diagnose it?

please help. '''