This doesn't necessarily mean a market meltdown is imminent, but it's important to know what to do -- or, to the point of today's topic, what not to do -- when equities prices are in free fall. Here's a challenge: Find a time when a major stock market index such as the S&P 500, Dow Jones Industrial Average, or Nasdaq-100 fell but didn't eventually recover. It might feel like stocks will never turn around during a market crash, however.