Imagine you had 60% in stocks and 40% in bonds. Your stocks went down 40% and your bonds went down 15%. A $100,000 portfolio now is worth $70,000, but even more out of wack is that now you are almost 50% stocks and 50% bonds. That's not what you wanted in the beginning, so sell/transfer/exchange some of your bond portfolio into your stocks to make it 60/40 again. You'll be buying stocks and lows and selling an asset class that did better. Buy low, sell high. That's what we're always trying to do.
I rebalance most portfolio's annually, so you should take a look at your funds/401k/403b and do the same. This market downturn will turn around eventually, but you don't just have to sit on your hands and do nothing.