Sometimes I'll walk away from it for about 2 or 3 months, so when I get back, it's almost like I'm have to get to know the characters and story just as if I was writing it for the first time. Or I'll read classical plays, because almost everything in some form came from classical plays, by reading them or atleast go over their rules it'll help me out.
There are some schools of thought that says that when you get a writers block that something in the material is not true to it self. Either there's a character that shouldn't be there, or a plot devise or a scene (s) that's out of the theme of the material that you're writing. With that in mind, I would go over your screenplay what in there that has, just HAS to be in there keep it, everything else, throw it away. M. Night threw away 3 or 4 drafts of the Sixth Sense before things started to click. These are things that have worked for me in the past.