This is from a well-known, NYTimes-best-selling author:
I am sitting here looking at a fic I have not touched since 2007. I have 135K done, including the last scene...or, about 2/3 of the total fic. I am ALSO sitting here looking at a novel that was due three years ago, for which I have something similar to an outline and the first 50K written (only 100K to go, right?)
I've been writing fanfic and profic since the 80s, and dealing with blocked, derailed, and MIA stories for most of that time. Here are some of the strategies that have worked for me. (NOTE: some of these ideas are mutually-exclusive, because every writer writes differently.)
1. WELCOME TO THE GULAG: Block out a specific time and place where you do the same thing every day: sit in front of the screen and make words come. Doesn't matter what you write, or even if you don't write. Just be there doing nothing else (no shopping, no reading AO3, no social media) for that one or two hours (no more) each and every day (same Bat-time, same Bat-channel). Eventually your brain gives up and you get to write what you want to write.
1A. If absolutely nothing else will come to your fingers, choose a favorite book (or longfic) and retype it.
2. FACE THE MUSIC: Between day job and commute (long) I was really bushed when Writing Time arrived in the evening. I just didn't have the energy—but I did have a deadline. Solution? ROCK'N'ROLL BAY-BEE!!! I wrote two novels to "Bad To The Bone". Just that one track. On infinite repeat. Loud. So pick a piece of music, declare it your writing music, and hit "Repeat" on iTunes.