system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Stock market crashes. How do they occur and once they do, is all money basically just gone with no way of getting it back?
gpt
Imagine you have a valuable collector's item, for example a home run baseball that was signed by some big name players.  Other fans have made you an offer to buy it, saying they'll give you $100 for it.  Personally you feel that it is more valuable and that they player that hit it is going to be an all-time great, and that someday when that player is in the Hall of Fame that the baseball will be worth $300.

Now imagine some new piece of information is revealed.  Maybe the player is found guilty of performance enhancing drugs, or his career turns out to be a total flop, maybe he loses his nerves on social media and becomes widely disliked.  Nobody cares about your baseball anymore and you have a hard time finding someone who would even buy it for $10.

The underlying value, growth prospects, and analysis you had done to previously calculate the value of the baseball is undermined.  It has simply become worth less.

Imagine you had a full collection of stocks like this, that you were planning your whole budget and life around, knowing that at any given time you could sell them all off at $300 per share.  You were planning to sell it all for your retirement someday, but the 'bubble' burst, and you realized that you'll hardly be able to get a fraction of what you had planned.  There wasn't necessarily any money that was 'lost', aside from the costs accrued in acquiring the security, but your net worth has dropped away.