We know one, and only one, thing about the stock market: It goes up. In the long run. Not in the short run. Not even, necessarily, in the medium run. But in the long run, it goes up. Oh, and we know one more thing about the stock market: you do not know when it will go up, or down. You are the "dumb money."
We learned these simple facts from the classic "only investing book you'll ever need," A Random Walk Down Wall Street, by Princeton economist Burton Malkiel. And lo! Burton Malkiel has descended from on high today to remind all of us idiots not to panic:
No one can tell you when the stock market will end its decline, but there are some things that we do know. Investors who have sold out their stocks at times when there have been very large declines in the market have invariably been wrong. We have abundant evidence that the average investor tends to put money into the market at or near the top and tends to sell out during periods of extreme decline and volatility. Over long periods of time, the U.S. equity market has provided generous average annual returns. But the average investor has earned substantially less than the market return, in part from bad timing decisions.
Buy and hold low-cost index funds. That is all that you do. You do not sell those funds when the market tanks. Buy. Hold. Do nothing else. We know not what tomorrow shall bring; yet we may be sure that when many seasons have come and gone, and the troubles of today have long been forgotten, our forbearance in these times of turmoil shall pay dividends that far exceed the present cost of standing up to our fears.
If you mess with your investments, you'll just fuck things up. Leave them alone.