if you take any given year, even five year period of this history of the stock market, you'll find a very mixed bag of returns. Some very high, some abysmally low.
But, there has NEVER been a 20 year period in the entire history of the stock market. Not 2008, not the great depression, not the collapse of the savings and loans, that has been negative. Not one. hold it for 20 years and you'll almost surely be in the black.
Doesn't mean it couldn't still happen in the future, but if you hold it long enough AND DON"T TOUCH IT, good things will happen to you.