For example, bitcoin has a market cap of $320B. At its peak it was worth about four times that. Did $960B just disappear? Basically, yes.
The coin has no intrinsic value (in the way that a can of corn does, for example). It's worth money because people say it is. And market cap is just a multiple of what it trades for at the margins times the number of shares (coins).
To give an example, say that I bought a bitcoin 10 years ago and that it was my only possession. At its peak, I could have sold the bitcoin for ~$64k, so I had a net worth of 64k. If I didn't sell it at that point and still hold it, I'm now worth ~16k. No one made $48K of of me... there were no transactions in that time period. The "wealth" has simply vanished.