What event brought the "party" of the 1920s to an end?
In the 1920s, a lot of people with limited understanding of stocks and bonds invested heavily and bought more stock than they could afford "on margin" (with mostly borrowed money). Stock prices inflated well beyond any actual value they might have ever had. On October 29, 1929, reality set in. The bubble burst and millions of people all over the world who thought they were millionaires on paper were suddenly quite destitute.
Many observers chalked this up to a market "correction" and thought the effects would be temporary as long as everybody kept their child-like faith in the infallibility of an unregulated market, but banks closed as people tried to withdraw their savings accounts. the depression continued for the better part of a decade.