Not everyone is a momentum/trend follower. There is a lot of capital out there that invests long-term on fundamentals.
Let's say Fidelity does some analysis and decides they would buy any company on the S&P500 at 20% under it's current price in a general market, non- company-specific black swan event. They can do this because they know that it would just be an over-reaction, those companies became a bargain, and they can afford to wait it out.
Then TRowPrice does similar research and they hear where Fidelity put their limit buys, so they put theirs at 19% under market to beat out the guys from Fidelity.
Then Vanguard does the same, and puts their limit buys at 18%..
..and so on until you have price discovery, even in a 9/11 type event.
Prices didn't go to zero at the start of WWI or WWII or 9/11 or at any other time, and they won't go to zero even if Obama came right out and said he was going to outlaw ownership of equities under penalty of death. Because a guy like me would take the risk and buy Chevron stock at $1 a share, then somebody else would outdo me and take a risk at $2 a share.. and so on up to price discovery.