The onset of The Great Depression loomed quietly through the latter years of the 1920's. Economically speaking the business cycle went through series of ups and downs. For example, wages did not keep up with the rising cost of living,and the distribution of wealth among Americans was severely unbalanced. These weaknesses combined with reckless investing in the stock market and with the lassiez- faire mentality of the U.S. government resulted in the 1929 stock market crash. Between 1929 and 1932 almost one quarter of the American workforce was unemployed. With the election of F.D.R. in 1932 the domestic policies of what he called a "New Deal" for America were put into motion. Although it would take several years for these policies to take hold and several congressional arguments to keep them in place, by 1935 Americans felt hopeful. The economy as a whole did not fully recover from the depression until after the attack on Pearl Harbor, which resulted in putting the American workforce into overdrive.
The Great Depression really was kicked off by the crash of the stock market in 1929. However, the real spiral downward started before then and the effects of the GD was felt even through the 1940s. Most books would classify the GD as having taken place in the 1930s when banks were rebuilding, jobs were being created by the government and generally people were struggling to make ends meet.
The Great Depression began October 1929 with the crash of the stock market and the worst years lasted until 1933 although historians believe that it actually lingered into the 1940's prior to the war. The loss of farm revenue and the failing of the banking system were largely responsible for the economic slide. Enotes has much more additional information at the following link!
It started just after the prospects of the Roaring Twenties. The then President Herbert Hoover, the popular conservating hero of WWI convinced American citizens that USA would have a booming economy and would become a prosperous country, but the disaster struck. On October 1929, also known as the "Black Tuesday", the stock market suddenly collapse like dominoes and everywhere was in chaos. Businesses and companies went bust and millions of people were jobless or were retrenched, or even their homes were lost. Economy was stagnating and other nation like Germany, France and Britain plunged into complete recession, and brought to their knees.
So during the Great Depression, many Americans resented that Hoover didn't provide any financial assistance, and so voted him out of office and so Democrat Theodore Roosevelt was the next President of USA. It started off with the New Deals, but it still cannot help USA to recover from the Great Depression. It is only during WWII in December 1941 did their economy rebounded.