The last one was particularly hard for me since I just retired. What did I do? The same thing I did with each crisis, which was nothing. I stayed invested, diversified, and continued to increase my retirement nest eggs through dollar cost averaging.
Many of my colleagues got out of the market after the 2008 crisis and never got back in. I followed these strategies for investing during a crisis and retired early while they are still working.
While it is better to sell before the crash than after, no one can predict exactly when. It may rally and gain massively before it crashes. It is better to stay in the game.