If the problem is that you simply don't know how you want to start, or you can't think of a clever opening or the tone you want to set - skip it and move onto the paragraphs. It's easy to fall into the trap of writing an introduction then strangling the rest of the essay by trying to stick to what you have said in that introduction. You're much more likely to write an interesting, lively essay if you start writing from your plan then really run with the ideas. Keep in mind that you have to make that content answer the question. By writing the introduction first you're clearly stating to yourself as well as to your reader what you're trying to do.
Introductions can do that.
You don't need examples, you don't need details - you just need to sweep the readers up into what you're going to do. Pause, then try reading from the beginning of the paragraph to regain your flow and then try to keep going. Try to relax. Now, if you can't get your flow back on that paragraph, leave some space, then skip onto the next one. Go back after you've finished and try that sticky paragraph again.