I find that when I ask my characters that question, the answers tend to throw me for a loop. So ever the clever one, I go "no, no, what do you really really want?" Then I'm in the drink. Swimming for my life. Up the creek, and every other cliche for a writer in trouble. Their answers never fail to scare me silly.
Usually, I have a pretty firm of idea of the answers before I start writing. I also know what happens in the last chapter before a word flashes on the screen. But when chapters start taking weird leaps and awkward pirouettes, I hustle back to basics. What is the reason for this chapter? How does it advance the story? What do we learn about the characters in it, that we didn't know before?
If those answers don't jolt me off the hot seat, I know it's time to dig deeper. A simple desire isn't enough to keep the story percolating. Why not? you may well ask. Because what makes a good book better is when you have characters with desperate, deep, devastating desires and you keep them away from whatever will save their souls. They can't have it. Won't even get close unless I relent and throw a bone in a weak moment.
So ask your characters what they would kill to get, take it away, lock it in a deep vault, and make them sweat emotional blood to unlock the box. That will get a book back on track.