Out of the box the LCD is usually too dim to show anything. One screw fixes it.
Before anything else
Because of how the LCD Keypad Shield is manufactured, there is a good chance the screen shows nothing at all when you first power it up, even with working code. That is not a fault.
Find the small blue potentiometer in the corner of the shield. Using a small screwdriver (or a fingernail), turn its screw clockwise . It takes several full turns before text starts to appear. Keep going until the characters are crisp, and back off a little if the whole screen goes dark.