wow, well, say something about yourself, ask who are you? What type of person you are, what is interesting about you, what can you convey to the adcoms that will ignite their interest? Who are you, what do you want to do, those are the two things you have to ask yourself, at least come up w/ an outline and sit on these thoughts for a while, it will make everything easier.
It's hard for me to give you any advice w/o you showign anything? The number question is what makes you click? After that ask, what is unique about you? It should all start to come together after that, you can pay closer attention to some of hte things I mentioned above once you start to get into the flow of the writing. Your topic isnt that important, its best if it talks about one of your accomplishments or highlights it in someway, but most important is getting a flow when you write, words should come naturally to you and also do not forget about the reflective part in these essays, so important. Thats what separates good essays from trite, medicore ones, even if you dont have alot of content to write about, do your best to tell a story, and then if you spend more time on the reflective level you might be able to answer alot of your own questions and add to the story you told above. Good luck man, i dont know how much that helped, but overall, just keep it simple, really ask reflective questions about yourself and dont just settle for saying, oh i have nothing, write it, make a note of it somewhere, however vague and useless it might seem, im sure you've heard this a million times before, but really you can come up w/ a rough draft, its just about being committed and saying im not leaving until i have something on paper. Good luck
btw, what is specific about a statemnet of purpose, is it just what you want to do, or is it also why you are a good fit for where you are applying?