If "Cyber Eltanin" is flipped face-down by an effect like "Book of Moon", then its effect of increasing its ATK is no longer applied. If "Cyber Eltanin" is later flipped face-up, then its ATK will be 0.
When Summoning "Cyber Eltanin", when removing from play LIGHTMachine-Type monsters on your side of the field, you only remove from play monsters that are face-up. If a LIGHTMachine-Type monster is face-down, then it is not removed from play.
You can Special Summon "Cyber Eltanin" by only removing from play all LIGHTMachine-Type monsters from your field if you do not have any in your Graveyard, or by only removing from play all LIGHTMachine-Type monsters from your Graveyard if you do not have any on your side of the field.