Either your hook -up is not compatible, which is probably the problem or your graphics chip is failing. Unhook it from the other screen and if the laptop screen works fine, then it's the hook-up with the other screen you are using. If it doesn't, you have a graphics chip problem. Kind of depends on how old the laptop is.
Why won't my laptop screen work? It is white with black across it with purple looking pixels surrounding the black. I have my laptop hooked up to a different screen with the HDMI cord. I just want my laptop screen to work- please help.
You can checkout the screen compatibility.It also depends that you have unhook another screen,If it still doesn't work then may be the IC resistors issue or being spoiled the display chip of screen.This PPI Calculator helps you to calculate your screen density.