Answer:
The cause was the expansion of cheap credit because of loose monetary policies by the Federal Reserve.
These policies led to an important increase in the money supply, that commercial banks took advantage of by issuing risky loans to people who had a shaky financial situations, and could default on their debts on the slightest economic contraction.
The effect was the Great Depression, when the asset price bubble caused by the policies of the Fed finally bursted out.
The Great Depression meant a sharp reduction in asset prices, the bankruptcy of thousands of companies, the lay off of millions of workers, and the loss of government revenue in the form of less taxes.