I think when a writer encounters a block, it's because something has gone wrong in the story. There's a hitch in the plot that prevents the writer from moving past it. So sometimes I'll back up, take a look at my most recent scenes and then back up even more, see if I took a wrong turn somewhere. Sometimes, I'll try writing the scene from another viewpoint to see if that makes a difference.
Another reason for writer's block is fear - emotional paralysis. Writers become afraid to write because they're afraid their story won't be good enough or they won't be good enough. Even after a writer sells a book, this kind of paralysis can attack as the writers wonders if they can do it again and again or if it was all just a fluke. The only way to get past fear is to face it and to write through it. Write anything and don't let yourself edit until you have at least several pages. Break through the internal critic and you'll find the story will usually start to flow again!