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.
Do this once and it stays set.