Many people who did business with banks rushed to the banks and attempted to withdraw their money. The problem is, they didn't have everyone's money. Banks only keep a limited amount of money on them at any time. Money put into savings accounts is used to lend to people. So, when people rushed and withdrew, the banks ran out of money and had to close. This coupled with people defaulting on loans made many banks fail, as they simply couldn't perform their function.
