I fixed this problem recently. The laptop screen was very dark, yet I could see everything on an external monitor (hdmi cable in this case) and if I used my lamp on the phone close to the screen, I could see everything.
I simply pulled out the battery cable, turned off the computer, then held the start button for 30 seconds (despite starting, keep it in, it will shutoff the computer after a while, and keep counting to 30 seconds from the starting point). Your battery should be empty by now. Attach the battery cable to the computer, and start the computer again.
I have no clue why it works, but it for sure saved me couple of dollars which the computer technician 100% would steal from me, worth to mention included a new part eg inverter.