I didn't know until about three days ago, what I was going to write. It just sort of popped in to my head as I was sipping my morning coffee and trying to wake up. Let me give you a few things to consider...
1) Have you had any vague ideas niggling at your brain lately?
2) Have you written any shorter works that a reader or reviewer has encouraged you to expand upon?
3) Have you had any dreams lately, that may have provided a spark of creative ideas or thoughts to you?