I'm going to share an ill-kept secret, I hit the "stuck" part in the beginning of the book.
I'm currently at that stage in the fourth book of my upcoming Urban Fantasy series (pre-orders go live for Book 1 at the end of the month!). I've spent a week with my wheels spinning in the muck. It's frustrating as hell. I know why it's happening; I don't have the right inciting incident. I write to chapter 5, then cut all of it, because it's still not right. The characters aren't on the right trajectory. Last night, just before my brain settled into slumber, I think I finally figured out the right opening. We'll see. Give me to the weekend to be certain.
In that short bit of snivel and whine, is my first tip to getting unstuck.
1. Keep Writing, but Try Different Perspectives
Put. Words. On. Page...a slight variation on what Jeffe suggested Sunday that still reinforces the habit and ritual of writing. Instead of forcing yourself to go forward and gain word-count, allow yourself to indulge in what-ifs as you rewrite the last scene that worked from a different state of mind for your character (or a different POV altogether if you're writing a multi-POV book). Something as small as changing the POV character's attitude/emotional state could unclog the blockage and help you roll into the next chapter.
2. Change a Core Element
If a scene just isn't working--and you're positive it's a necessary scene--change a core element. Change the setting, the characters surrounding the protagonist, the nature of the challenge, or the means by which your protag achieves the goal of that scene. Maybe what you thought should be a physical battle works better as a battle of wits. Maybe a private scene should be very public. Maybe you need to change the weather. Hey, you're God of your world. Make your characters act out/up during a wicked thunderstorm. Not everything has to happen on a sunny day or a foggy night.
Take an hour to boost the oxygen flowing through your system. Whatever your level of preferred exertion might be--Ashtanga yoga, rock climbing, stroll around the neighborhood--whatever gets you up and moving. Go without the earphones. Skip the playlists, the phone calls, the other distractions. Just you, your brain, and your body; reconnect with yourself. Might be surprised by the sort of ideas that'll come to you...some might even be relevant to your WiP.