what's it worth?

     A thing offered for sale is worth whatever you can get for it--but not until after you get it. You can offer an Elvis commemorative plate for $500--but if you only get $50, then the plate is only worth $50. This would be the "market price" of your commemorative plate. If you think that accepting the offer of $50 is losing money, you will have to hold onto your plate until someone offers more money--even if that takes years.
     The stock market works the same way. People "gain" and "lose" money every time the price of stock changes--in theory. But they haven't really gained or lost any money until they have sold their stock at a price that makes money for them, or loses it. The people who owned stock at the beginning of the great depression didn't all lose money. If they could afford to hold onto the stock until prices rose again--which during the depression meant several years--they didn't lose any money at all. Some of the investments in firms that went bankrupt would have been lost--but nothing else. The "panic" sellers during the depression had bought the stock "on margin"--which means on credit. When it dropped in price, they had to sell--they no longer had stock in an amount equal to what they had borrowed to get it. They had to sell or make up the difference in price in cash--if they had it. Market prices "going down" don't have to mean losing money on investments.