Nowhere. You have to have money to have money. If you trade your money for stocks you have stocks, not money.

Let's change it to anything but stocks and it starts to make more sense.

Suppose I buy a gold nugget for $100 based on the current market rate. Can I pay for a meal with it? No. Can I buy groceries with it? No. It's gold. I either have to find a person willing to trade with me for it (and then 'value' gets weird) or I have to sell it to someone so I can have money. I had $100, now I have gold.

If the price of gold goes up AND I can find someone who will buy my gold, then I can sell it to them. Now I have money, but I do not have gold. The money didn't appear by magic: they had to have it first and how they made it isn't my business.

If the price of gold goes down, my $100 is still in someone else's hands and I still have my gold. Nothing "went away". But now it's harder for me to find someone who will give me $100 back for the gold. If I find someone that gives me $95, I have less money than when I started, but the money didn't "go away". The person I originally gave $100 to still has it. I traded it to someone else for a different amount of money.

If you can't find someone to make the trade, then you technically haven't "made" or "lost" anything. You've still got exactly what you traded for. It's just not money and you can't find people who want to trade it.

That's why some people argue billionaires don't really have all of their money. If someone has $100m worth of some stock, they have stocks, not money. If they try to sell all of it at the same time, they might have a problem: usually the market for any stock isn't big enough to satisfy that kind of sale. So they can only sell some of it, and if investors hear they're trying to sell ALL of it they might get scared and stop buying. Then they can sell less of it AND people are willing to buy it for less.

Same thing with if I invest $100m into gold. I traded money for gold. That gold will probably go up in value, but I can't buy a hamburger with gold very easily. I have to trade gold for money so I can trade money for the hamburger. And there probably aren't a lot of people who want to buy $100m worth of gold all at once. So "How much money do I have?" is a very complicated question in that situation, but we can start by saying simply that I have no money, because I traded it all for gold. How much money can I get for that gold? That changes every minute based on how many people want to buy gold, how much gold they want, and how much they will pay. Sometimes the answer is lower, sometimes it's higher.

But the money I traded didn't go away or create more. It just went to a different person.