After World War I, Europe was much poorer than it had been before the war. Many men had been killed in the war, and many houses and factories had been ruined. Then in 1929, there was an economic crisis too. A lot of companies went out of business, and then the people who worked for those companies were out of work.
Once people were out of work they spent less money in stores and restaurants, and many other companies went out of business because there weren't enough customers.
To find out more about the depression, check out these books from Amazon or from your local library: