a blank page

I do a lot of plotting in my head, and I get all excited about the characters and the ideas and the concepts, and the little bits and bobs, where I think, oh, yes, I should put that in there.  So how come when I am faced with a blank page, that I end up looking at it blankly?  There should be some easy way to download thoughts to the screen!  Snap, snap, engineers! Get crackin'.   (Of course, that's a whole genre of futuristic horror stories, right there!) Ha!