Oh my gosh! What a nightmare this would be! I'm made of sunshine, so I can't even imagine a world without it.
Was it a sinister plot?
Was it the world simply getting old?
Did the moon decide to steal all the glory?
Man oh man... this would be awful.
I love to write in a more nightmarish style. As for your other questions - you decide! The moon could have wanted to have full reign and therefore must kick the sun out, the moon could have not liked the sun, evil magic caused this, etc etc. I like the idea that a creature from a different story (any of your choosing) that is really evil decided to cause chaos in the world. =)
Any way it plays out, that would be a sad world indeed - even though the night is beautiful!