What brought about the Great Economic Depression?

There was some financial stability between 1924 and 1928. Short term loans from the USA helped industrial recovery in Germany. In 1929, when the Wall Street Exchange crushed, the US withdrew all the loans. People rushed in to sell their shares as they thought that the prices would go down drastically. It was said that on the 24^th October alone about 13 million shares were sold. This brought about the Great Economic Depression. The national income of the US fell by nearly 50% between 1929 and 1932. The factories were shut down, exports fell, farmers were affected badly and the speculators withdrew their money from the market. All these affected not only the US but the entire world.