it reduces to half . C = Eo * A/ d and E = 1/2 C*V^2. there fore when d = 2d. the capacitance reduces and there by energy reduces.

This energy loss can be explained in the following way:

when you are pulling the plates apart the capacitor tries to oppose this.So,it does work to keep the plates together which you have to overcome to pull the plates apart.Since it is doing some work against your applied force,the energy stored on it decreases.

But when distance b/w 2 plates is doubled.....V will also double na?

It depends upon weather the plate is connected to the battery or disconnected after charging.