What happened to her after she was locked up with the ghost ??? How was Virginia locked up with the ghosht.. The ghost is already dead... Isn't it ?

Sir Simon is dead and it is his ghost that we encounter in the story. Until the ghost too is relieved from the ties of this world, Sir Simon too cannot 'rest in peace.' 

Moreover, the ghost has been given many human characteristics in the story. Thus, Virginia being locked up with the ghost can be explained. But, what happened to her after being locked up remains a mystery in the novel.