I was at Amzn early '00s when we lost 95% of our market cap. Later at FB I negotiated a down-round in '09, and then in '12 our stock dropped 50% post-IPO. I was on the board of a public company that went bankrupt (Borders) and a start-up that went under (Hello). Some lessons:
When startups start taking off, the founders usually can't tell what's changed. It's some combination of all the random things they did to make it take off, but it's hard to say which ones.