Take a walk. Do something completely different. Set it aside for a while and come back to it ... my brain will still chew on a story while I'm doing something else. Watch or read something else -- it might trigger an idea. Do a complete character history of your hero and heroine and some of the minor characters as well so your brain has material to work with. As scenes come to mind, jot them down (I use MS OneNote) even if they're not in any particular order, you use just bullet points or flesh it out a bit to work in later.