For those out there wondering what to do:
First off, if we are talking about your retirement funds, and you are more than 10 years from retirement, do nothing. You might not want to hear this, but the time to sell is passed. We might still have legs down to go, but the ability to guess market bottoms to buy back in at is notoriously impossible, and odds are you will be wrong and miss a large piece of the rebound and that return. Just sit tight and let it ride. Forget it is there.
In general, if you are close to, or in retirement, you should already be largely in cash and bond type funds in order to protect your cash value. Don't change that. The more you depend on those funds, or the sooner it you will need it, the less market exposure and more cash you should have.
If you are looking for investment opportunities with spare cash, look for industry leaders. Things which got beat up in these conditions, but have business models to survive these conditions. Banks, brokers, and exchanges are my typical favorites, but YMMV. There could still be volatility and big legs down, but if you are looking longer term to add to your portfolio, this is a good time. Blue chips first and foremost.
As for day trading type stuff, this is a crazy dangerous times. The moves are wild, and predicting the next seconds moves is a fools errand. One tweet can blow up anything. That goes for stocks, options, bonds, crypto, etc.