The issue may be that the external screen swapped back to the laptop screen. If you can see an image on the LCD but it's dim, it may not be the motherboard or the video card but either the inverter or backlight on the screen. Run the laptop only on the external monitor. Turn it on, and soon as you do that, close the lid. You'll need an external keyboard and mouse to use it then. The laptop should see that the monitor is the primary display and send the image to it. See if it stays on. If it does, you need to look into repairing your screen, an inverter is cheap and pretty quick, a backlight or a new screen you may want to look into getting another laptop then.