Have you tried doing a brain dump?
If you don't know what this is (apologies if you do), take a piece of A4 paper and turn it so that it's landscape rather than portrait. Write the essay topic in a bubble in the middle of the page, then surrounding this bubble just jot down key themes that your essay might include. The point of this is to do it quickly, without thinking too hard, so that you can access everything that's in your head.
So, for example, if your essay was about Mumsnet (how easy that would be!), then write Mumsnet in the middle, then you might have, say, 6 different themes going round this main bubble - e.g. Chat, AIBU, Recipes, etc.
Then starting with the first bubble, just down a word/key thought that you associate with that theme, draw a small line away from it and then jot down the next word/key thought on that bubble - keep going until you can't think of anything else and then move on to the next bubble.
You may suddenly think of another theme, which is fine, and you may realise there are links between themes/thoughts - which is also fine, as it will give you the beginning of a structure for your essay.
You should do this as quickly as you can and should try to fill the page. Once you've done it, step back and look at what you've written, and it will help you to find your arguments, counter arguments and a way in which you can structure the essay.
Good luck and hope this helps a bit!