There's a lot of great advice already listed above, but this point at the end of Graeme's is along the lines of what I was going to add.
I'm a bit of an animist, and I think stories have souls/spirits of their own. Maybe they're just an extension of your own soul, I don't know for certain, but I try to treat my stories like living creatures. Your story will want to go a certain direction, based off the life you've breathed into the characters, and it may not be the same direction you intended to go from the beginning. Trying to force my characters to go down my outlined path just because I'm the boss is often what gives me writer's block. Usually, when I give them a little more rein, they take me down paths I wouldn't have thought of, and make my entire experience as a writer that much more enriching.
But that's me speaking as an outliner who has had to learn how to nurture his plots so they grow naturally. If you're the kind of person who has found their strength in spontaneous writing, then maybe what you need is to add a little structure back in so your story can have something to grow into. The balance between structure and creative liberty are what allow a writer to write. Some require more structure and some require more creative liberty. If you're focused one way, try adding a little of the opposite and see what happens.