This is the worst! Or when you have ideas in your head when you're driving or half asleep or in another situation where you can't write and then they're gone by the time you open a word doc.

One thing I find helpful is to go for a walk and let myself think about the story, fully conscious and undistracted, but unable to write. So I just gotta think about what's going on with the characters, how they feel, what they're thinking. It helps me get over a hump when I'm stuck in the middle of a story.... but it never really helps to get something started when I'm not sure where to start.