Use two hands when you close your laptop. If you use just one hand and you pull it shut by grabbing one of the corners of the screen you are slowly but surely damaging the cable that connects the screen to the rest of the computer. It puts a bending, twisting force on the screen and eventually, the cable will break, and the screen will either be scrambled or dark.
Prevent this problem by closing your laptop with both hands, one on each side of the screen.
If you must do it one-handed grab the center of the screen, not the left or right top corner.