Help With Story Writing?

So, I'm trying to write this story on Wattpad. I've had the plot written out for a while now, and I've done short summaries of what I think should happen in my first five chapters. Up until the point where I begin to write, my mind just goes blank. It's not like I'm a bad writer or anything, I'm actually quite good. (I'm not bragging; I've personally been told this.) This has only happened once before but that was when I was trying to write a really crappy story. I ditched the story in the end. I don't want to do that with this idea though because I've been working on it for ages! Please, please help me! I badly want to write this story!