The Screen Is Dark and the Button Doesn't Light Up
The flat cables (EXP1 and EXP2) are probably swapped. Open the cover and switch they way they're plugged in.
The Screen Is Dark but the Button Is Lit Up
The ribbon cable for the screen is probably not completely secure. Open the cover and remove the board. Remove and then re-secure the ribbon cable.
The Button Moves but Doesn't Change the Option on-Screen
A. The knob housing may be loose. Try and rotate the clear housing. If it moves, remove the knob front and tighten the hex nut.
B. The knob front may not be seated on the post. Depress the button completely.