Should I buy a single stock?
You can definitely get rich picking the right stock. You can also definitely lose all of your money picking the wrong one. As an equity owner (stock=equity), if the company goes out of business, your shares expire worthless. Your investment goes to zero.
Diversified investments are great options if you don’t have the time to monitor an individual investment. You can buy an index fund or ETF (Exchange-Traded Fund) that represents the 500 biggest US companies. One big investment company buys all the stocks, then you just buy a share of their pool. For something like the S&P 500 to go to $0, that would mean the US economy would disappear. So average market returns won’t make you a millionaire overnight, but you’ll go up over time and certainly never go to $0.