I'd like to say it never happens, but it does. I typically will go back in a story and re-read what I wrote last to help kick-start an idea or point me in the right direction. If that doesn't work, I change the music channel and try to ride it out, or head out for a mind-clearing walk. As a last resort, I pick up a deck of Tarot cards, randomly draw three cards and read the card's explanation from the descriptive book that came in the box. That usually works quite well for me.