Stories can take on a life of their own. Sometimes that's a good thing. Occasionally, it's annoying as hell.
I've been working on my next novel, After Dark, but this other story keeps interrupting and demanding that I write it now. First. It refuses to wait.
I try to ignore it. No good.
I make some notes and tell myself I'll get back to it later. No good.
I try to sleep and pieces of it, hints of what it could be flash through my mind. All I can do at this point is put After Dark on hold and write this other intrusive bastard of a story.
Either that. or let it drive me nuts.