So today one guy, one trader was charged with causing the flash crash in 2010. That one guy can cause that kind of damage is amazing. See http://www.cnbc.com/id/102573733.
Bottom line though, is that the DJIA was approximately 10000 that crazy day and if you panicked, you sold at a terrible time, the DJIA in now approximately 18000.
I was very young in the market in 1987 when the market plunged 22% in one day and truly thought the world was over. But as I learned over the years, again, if you panicked and sold, you look pretty silly right now, the DJIA was 1800 then. Link here, see http://en.wikipedia.org/wiki/Black_Monday_(1987). The DJIA has risen 10x since then!
The point is that traders may care alot about intraday moves and that is unfortunately who you see on the news so it is natural to get likewise scared but learn from these moves that they are temporary and in most cases, awesome buying opportunities for long term buy and hold investors.
Remember, passive investing beats active investing. Don't get rattled by intraday and short term moves. Buy quality and let others sweat it out.