The "crash" that Harper Lee is referring to is the Stock Market Crash of 1929 that helped the economic downslide to the Great Depression of the 1930's. It all started on Black Thursday when over 16 million shares on the New York Stock Exchange were sold in a single day. During this time, unemployment reached over 30% with close to 15 million Americans out of work. At the same time, over half of the banks in America failed. If you have ever seen the movie, It's a Wonderful Life, a Christmas classic, it chronicles the near collapse of a bank in Seneca Falls, NY.
For the people of a poor, southern town like Maycomb, they would have been especially hit because of its lack of industry and jobs. Harper Lee mentions at the beginning of the novel that the people had, "Nothing to buy and no money to buy it with." The depression would have especially hit the poorest whites and the black members of the community because of the lack of jobs. Jobs would have been given to whites before blacks in town. This is also why Mr. Cunningham pays Atticus for his services with food. He probably didn't have the money to pay him. This is a time in our history when a lot of people lost everything they had because their money was tied up in banks. It was an era of fear, and one reason why Harper Lee included a reference to Roosevelt's famous line from his inaugural speech, "We have nothing to fear but fear itself" in the novel's exposition.