What do you do when you have writers block?

I've gotta write a paper but I'm getting some total writers block. I can imagine what I want to say but actually sitting down and writing it seems more of a task. I'm a little weary too only because last time I wrote a paper for this professor I got some pretty bad criticism (on a paper I took a good deal in doing...) so I guess the other problem is that I'm a little intimidated. Any help would be great! Thanks