Great question! I either switch to another project for a bit, or I sit and daydream with purpose. What I mean by that is purposefully visualizing scenes with your characters. It might not even be a scene in the book. It can be something simple like figuring out what your character would order at Starbucks. Or what they would say to a friend in a certain situation. Just trying to envision your character's quirks and so on. If you don't know your characters if they aren't speaking to you, you get stuck.