The first rule of writing is Don't Panic!
When a writer gets stuck on something, she can sometimes panic, and obsess over what she's stuck on. Although it's sometimes not easy to apply, there's an simple remedy for getting stuck: work on something else. If you're stuck on the hook or introduction, work on the thesis statement. If you're stuck on the thesis statement, work on summarizing your research. If you're stuck on the argument, just write what you think. Remember, you do not have to compose an essay in the same order that it will be read. You can start anywhere and jump around as you please.
There are a few elements of an essay that don't absolutely have to be terrific, notably the hook and the conclusion. You might lose a couple of points, but I don't think anyone ever got a B on what would otherwise be an A paper just because the hook was weak or the conclusion stereotypical. If these elements are bothering you, you can leave them until near the end of the writing process; if you run out of time, you can just phone them in.
You need to end up with a thesis statement, and it eventually needs to be good, but you don't need to start with a good thesis statement. The thesis statement is a one- or two-sentence declaration of your position. If you're having trouble with your thesis statement, you can try starting with something vague: "I agree with Ralph Nader." "I disagree with George W. Bush." "Global warming is bad." "The gold standard is good." You can even go with something equivocal: "I'm not sure whether we should or should not teach New Math to elementary school students." You'll want to revise your thesis statement later, but some focus is better than no focus at all.
If you're really stuck, there's a lot you can do without any sort of thesis statement at all. You should have at least a topic, so you can start by just writing down what you think about that topic. Don't worry about spelling, grammar, punctuation. Don't worry about structure, organization, or coherence. If you can do nothing else, getting words down on paper that are even vaguely related to your topic is better than a blank page.
Writing is thinking: the act of translating your mental state to written words on paper or on the computer actually changes what you think of the subject matter. If you find yourself not writing, write something. Anything!