Why was it overvalued (according to which metrics) and what is fair value?
Overvalued compared to historical data, such as all the usual ratios (debt to equity, P/E, Shiller PE). Of course this brings the old question - are indexes overvalued when almost everything else is overvalued as well? There is no such thing as "fair value", sure you may calculate Graham fair value or a similar number but it won't say much because it will be high as well.
It is a difficult time, in the past 10-15 years growth outperformed "value" investing. Also U.S. equities outperformed emerging markets. It may be the exact opposite in the near future, or it may not.
It is also possible that even bigger collapse will follow this stock crash - perhaps collapse of the junk bonds, or problems with USD liquidity (eurodollar deposits and USD-denominated debt), or maybe collapse of central banks.
I thought I was pretty well diversified and immune but I wasn't and I think very few people were (should've read
@Martin Everson 's posts here hahaha). Basically everything is "deleveraging" and losing value - stocks, bond yields, the euro, soon the dollar, bitcoin...
By the way - all the "deposit protections" from FDIC, from EU banks - they were created for small occasional defaults. A small regional savings bank goes bankrupt and FDIC is ready to cover the losses. However, it isn't prepared for a total collapse.
There are some possible scenarios...
- everything will return to normal, world will reopen and equities reach new highs in 6-12 months thanks to central banks' actions
- deflation and depression will come
- deflation will come followed by hyperinflation
- some crazy scenario such as China or Bitcoin taking over the world
If you were sure they were gonna tank, why didn't you profit from it? There's a thing called put your money where your mouth is.
Answering for someone else but... the crash was pretty fast, maybe fastest in stock market history ever. Even those who predicted such thing and those operating hedge funds (e.g. Ray Dalio) got it wrong and weren't prepared.