Hi Mate,
I have a lenovo G40, for some reason my laptop screen went very dim, you need a flashlight to see what is happening.
but when you connect it to a extended monitor, the extended monitor will work well but my laptop screen is still not visible?
anyone experience this? thanks 
Sounds like the backlight on the display might be faulty.
(Or you turned the brightness down all the way…)
Try booting with a Linux live USB, if that looks ok then it’s software related, if not then it’s almost certainly hardware.
If it’s hardware then sometimes you can replace just the backlight, sometimes you have to replace the whole LCD panel.
yeah i think. it is a hardware issue.
thanks mate!
My guess is that your back light is broken and not your display itself.