The Great Depression had unbelievable effects on not just the United States but the entire worlds population! People were left homeless, without a single penny. Most people were starved because they could not afford to eat for many days at a time. The whole world was in an economic crisis because of what was happening in the US.
The unfortuante people who kept their money and their savings in the banks had lost all of the money that they had trusted the banks with. As the United States grew out of the depression and got back on its feet, some people were so badly effected by the situation of the banks losing their money, that they no longer trusted them. They began hiding all of the money they had instead of risking losing it a second time if the great depression would somehow repeat itself.