Oddly this is a question I receive from readers and interested parties as well as one I ask myself.
I usually stammer a bit and try to think of an answer quickly so I don't look like a writer that hasn't a clue. But sometimes, we really don't. There are those stories that wake you up in the middle of the night or the scenes that come to you in the shower. They are tiny flashes of something so much bigger.
Then there are the times, for me usually when I've committed to a project or a deadline, that the "muse" disappears. Though I don't believe this muse is something outside myself. It's years of reading, watching, imagining. Our brains always ticking away with the stories we've seen and the stories we've kept going in our minds.
As I've said before, I have three finished books that will be out in August. That doesn't mean I'm not working, there are always more books to write. Some times I need to take a break, turn off the little hamster in my head and not dream of new romances.
A short break often brings back the "muse". I have a story plotted, as I stare at the screen I wait to hear the voices start to tell me their tale. I really hope it's a good one.