Tens of billions worth of Bitcoin have been locked by people who forgot their key.

Of the existing 18.5 million Bitcoin, around 20 percent — currently worth around $140 billion — appear to be in lost or otherwise stranded wallets, according to the cryptocurrency data firm Chainalysis. Wallet Recovery Services, a business that helps find lost digital keys, said it had received 70 requests a day from people who wanted help recovering their riches, three times the number of a month ago.

The cryptocurrency’s unusual nature has meant that many people are locked out of their Bitcoin fortunes as a result of lost or forgotten keys. They have been forced to watch, helpless, as the price has risen and fallen sharply, unable to cash in on their digital wealth.

Bitcoin owners who are locked out of their wallets speak of endless days and nights of frustration as they have tried to get access to their fortunes. Many have owned the coins since Bitcoin’s early days a decade ago, when no one had confidence that the tokens would be worth anything.

The quandary is a stark reminder of Bitcoin’s unusual technological underpinnings, which set it apart from normal money and give it some of its most vaunted — and riskiest — qualities. With traditional bank accounts and online wallets, banks like Wells Fargo and other financial companies like PayPal can provide people the passwords to their accounts or reset lost passwords.

Bitcoin has no company to provide or store passwords. But the structure of this system did not account for just how bad people can be at remembering and securing their passwords.

“Even sophisticated investors have been completely incapable of doing any kind of management of private keys,� said Diogo Monica, a co-founder of a start-up called Anchorage, which helps companies handle cryptocurrency security. Mr. Monica started the company in 2017 after helping a hedge fund regain access to one of its Bitcoin wallets.