It is extremely difficult to know what to put in your concluding paragraph without knowing what your thesis statement for your essay as a whole was and without knowing what sorts of arguments you have made in your essay. This is because a conclusion is generally meant to reinforce your thesis and the main points of your essay.
Therefore, it would be important to know what the topic of the essay was. Were you supposed to say which cause of the Depression was most important? Were you just supposed to describe the most important causes? It’s also important to know what you said as a thesis. Did you say something like “In this paper I will describe the main causes of the Depression” or did you take a stand like “The main causes of the Depression were the stock market and overproduction?”
In general, what you want to do is to sum up the main points of your paper. You do not want to bring in any new arguments or any new facts. Instead, you just want to reiterate the most important points. You might want to include one idea from each of your body paragraphs. Ideally, you would do so in a way that fits with your thesis statement.
Here is an example which may or may not fit with what you have written in your essay:
- The Great Depression had many causes. It was caused in part by problems in the farm economy and in part by excessive production in the industrial sector. These problems caused weaknesses in the foundations of the US economy. When the stock market crashed, the weaknesses were exposed and the Depression began.