What would you do in a "lost decade"?

Historically, there have been a few "lost decades" for stocks - periods of 10 to 20 years where stocks didn't grow at all. The most recent one was the dot-com bubble and GFC, when it took the stock market about 13 years to recover, and that's not even accounting for inflation. We're so used to the stock market giving us great returns year after year that I'm not sure how I'd react if it had 2-3 major crashes one after another and ended up with zero growth over, say, 15 years. It must be psycholo