Writer's block takes many forms. Sometimes you just lose interest in a story, or writing in general. Maybe you suffer from a lack of focus, or you just feel the well of ideas has run dry. There's an easy way to find your motivation again, and get yourself back to writing.
Writing for Writing's Sake
When you're having trouble writing, it's often because too many thoughts are getting in the way. Over-thinking it is a common problem, and it's also common to feel sort of "blah" about writing. When you need to motivate yourself, think about your favorite movie.
What's your favorite scene from the movie that you like best? If it's really your favorite, chances are good that you know at least one scene backwards and forwards. So, write it. Describe the scene with your words, using as much of the real dialogue as you can remember. Think about every little detail, and put it on the page. Just start writing it, and get into the rhythm of punching the keyboard. Notice how nicely the words flow. It's not your scene, but they are your words. So write out the scene and go back and re-read it.
When you're done reading, you may find that you're ready to write something else. Maybe it's time to dust off one of those neglected stories, or maybe there's a new idea in your head all of a sudden. Try it, and see what happens.