The feeling of being stuck, or blocked, affects everybody at some stage or other of the writing process. Although frustrating, writer's block, when it doesn't last too long, can be a very creative state; an important part of reflective practice is to recognize that you might be feeling blocked because you are encountering a genuine difficulty or problem which you haven't thought about enough. This is particularly likely to be the case when you have a block about beginning the essay, when you probably need to consider the essay's rhetorical situation, and also whether you understand the expectations attached to the essay.
It is a good idea to analyze why you are feeling blocked, because the underlying causes of your block won't just go away of their own accord. Even if you manage to write the essay in a sudden burst of activity, your writing is still likely to show the effects of the block through a scatter-gun approach, and be full of unconnected insights which aren't properly explained. For this reason, it is never a good idea to rely on inspiration to write an academic essay.
A common reason for feeling blocked is that your own assumptions about what you can achieve are unrealistic. This might be helped by considering the nature of your position as a writer; it also often happens that the very ambition to create a perfect piece of writing is what is holding you back. Nobody gets everything right on a first draft, and there is no such thing as perfect writing in any case - it has often been said that a piece of writing is never finished, only abandoned.