When readers ask this question, all they have to do to get the answer is turn the page. When writers ask this question, much more work is in store to find out. Sometimes you get to a point in a story where you've bled your ideas and have no clue where to go from there. Before you stall or move on to another project, try these two techniques to push your story out of the doldrums.
Go Back to the Original Idea
Sometimes to go forward, it helps to go back to the original concept. What got you to this point in the first place? A character? A scene? A snippet of dialogue? Revisit those first images or pages and try to remember your initial feelings about them. What about the idea drove you to write the story? What kept it moving? If you're not sure, go back and analyze. There is a reason you started telling this story. And that may very well help you to figure out where it needs to go next.
Talk to Your Characters
Nothing happens without the characters. When all else fails, go back to them. Spend some time journaling from their points-of-view, interview them, write scenes and conversations that have nothing to do with the story. Let them live in your imagination. Watch them go about daily life and experience interactions and conflicts outside the main story. Your characters are leading anyway so let who they are and what they do lead you to your next move.
What happens next may not be as far off as it feels. Keep writing, thinking, and imagining and you will unleash the next chapter in your story.
Spend a writing session journaling from the POV of your main character. If you've never tried this, have your character talk about the story to get going. Let him/her ramble. It may take more than a page but stick with it and you will unearth something new and possibly unexpected that may just spark an idea for what happens next.