Back in the Good Old Days, I was an active stock market investor/trader. I was also a fan of Louis Rukyser's Wall Street Week on PBS. On Friday, October 23, 1987, after the Crash the previous Monday, he had a lot of sage advice. You can see it here, beginning about the 1:45 mark. The next minute or so is particularly germane. Remember that the market had dropped the previous week about the same relative amount that the current market has dropped in the last year.
Compare Rukyser with Jim Cramer's advice here ( beginning about a minute in). Cramer is wrong, particularly in light of the fact that all stock market investors should already have a year's reserve outside the market. If you do, you don't need to panic today at all. You have a whole year to worry before you need to panic. Time will tell whether selling now is good advice.