When did the international economy collapse and why?

The Great Depression started in 1929, when the Wall St Market in America crashed. It incurred after the Roaring Twenties', which was when almost everyone was making great economic gain, mostly from buying stocks and shares. During the 20s, there was a lot of supply of goods and services, with low demand, which resulted in the decrease in the value of goods and services. Due to the depreciation of the value of objects, people thought it was cheap, often spending more than they earned, borrowing money from the banks. However, after 10 years of economic success, loans were not paid off, not only by individuals, but by countries as well, resulting in the Great Depression, where there was a lack of money circulating and everyone in huge debts. 

However, the Great Depression did not affect everyone. Rich people actually made money while poorer people and people from the middle class were affected. It took about 10 years for the economy to recover.