I replaced the lcd screen, the touchpad, and the keyboard on my Dell Insipron 9400. The touchpad and the keyboard work fine, but for some reason the LCD screen will not display anything, not even at startup. I tried putting the old LCD back in, but it doesn't display anything either, so that rules out the new LCD being defective. I even started over and checked for any loose cables and I found none. I can hook up an external monitor and that works just fine. In fact, that's how I'm using the laptop presently. So what could be the problem?