Okay, here's the situation. I've just come back from a fun, but busy, weekend out of town. I returned to a bunch of laundry, suitcases to unpack, school forms to sign and return, a child with a nasty cold, a Monday morning doctor checkup...oh, and I pulled my neck somehow when unloading the car. Sigh. BUT--amazingly enough I was able to scrape together a page of my manuscript yesterday.
The reason yesterday's writing was easy was because I already knew what I was going to write. I was actually in the middle of the denouement scene (no, I'm not done with the book. But I knew how I wanted it to end.). Because I knew where I was picking up and what was going to happen in the scene (I could see the action happening in my head), it made it much easier to write. I had a roadmap for writing that day.
Here's one way I've found to easily hop into your writing with no delay on a day where you might not have time to figure out which scene you need to work on.
First of all, I have a folder in "My Documents" labeled with my book's name. Inside that folder, I have many different saved documents. You can plan ahead the scenes you'll need to write in the future. With a genre mystery (cozy, police procedural, etc), this is pretty easy. You know you need a scene where the first body is discovered. You know you need a scene where each of the 4 or 5 suspects is interviewed about their opportunity and possible motive (that's 4 or 5 separate scenes right there). You know you need to plant clues and red herrings. You know you may need a scene where a second body is discovered. Possibly a scene where your protagonist is in danger and a scene where the killer is apprehended. You see what I mean.
I've already named and explored the character of each of my five suspects. So one day I created five separate Word documents. Each one had the name of my sleuth (Myrtle) and "interviews ________(suspect's name)."
On a day where you just need to jump right into your manuscript and write it, having your book divided into its components makes it easy. You can even print out the document if you're on the go and take it with you to scribble on while you're out. You can write at the top of the page some notes for yourself on what you want to accomplish with the scene ("have suspect lie about their alibi" or "have suspect implicate another character", etc.)
Later you can assemble your book into one manuscript by copy/pasting it in to approximately the right place. Once you print it and read it through, it's easy to add scene transitions, segues, etc.