Many banks had lent money to stock speculators.
Many banks had invested depositors' money in the stock market, hoping for high returns that they could get by using the money for conventional loans.
When the stock market crashed, banks lost money on their investments, and the speculators defaulted on their loans.
Banks then cut back drastically on their loans which in turn affected consumers.
Banks during this time were not insured by the government. If a bank went under, then everyone's saving in the bank were wiped out.
As word spread about bank failures, people made a run on banks to get their money.
Banks make profits by lending money received from depositors and collecting interest on leans.
People making runs on banks discovered that their savings no longer existed.
This run on banks led to over 3,000 banks collapsing over the first two years of the Great Depression