As the FTX cryptocurrency exchange imploded last fall, Tom Brady, the seven-time Super Bowl-winning quarterback, made an urgent phone call.

He dialed Sina Nader, FTX’s head of partnerships. The exchange’s staff was in the middle of a crisis meeting with its beleaguered founder, Sam Bankman-Fried. Mr. Nader couldn’t answer. “I never would’ve expected to decline a call from Tom Brady,” he said.

Mr. Brady had reasons to be concerned. As an “ambassador” for FTX, he had appeared at the company’s conference in the Bahamas and in TV commercials that promoted the exchange as “the most trusted” institution in the loosely regulated world of crypto.

His money was also at stake. As part of an endorsement agreement Mr. Brady signed in 2021, FTX had paid him $30 million, a deal that consisted almost entirely of FTX stock, three people with knowledge of the contract said. Mr. Brady’s wife at the time, the supermodel Gisele Bündchen, was paid $18 million in FTX stock, one of the people said.

Now FTX is bankrupt, and Mr. Bankman-Fried is facing criminal fraud charges. Mr. Brady, 45, and Ms. Bündchen, 42, have been sued by a group of FTX customers seeking compensation from the celebrities who endorsed the exchange. On top of it all, the terms of the deal would have required the former couple, who divorced last year, to pay taxes on at least some of their now worthless FTX stock, two people familiar with the endorsement deal said.

Their situation is the highest-profile example of a humiliating reckoning facing the actors, athletes and other celebrities who rushed to embrace the easy money and online hype of cryptocurrencies. During the boom times, Paris Hilton, Snoop Dogg, Reese Witherspoon and Matt Damon all gushed about or invested in crypto projects, bringing a mainstream audience to the wonky world of digital currencies. It was fun — and lucrative — while prices soared.

But last year’s crash ended the celebrity crypto bonanza.

In October, the Securities and Exchange Commission ordered Kim Kardashian to pay $1.26 million for failing to make adequate disclosures when she endorsed the EthereumMax crypto token. In December, a lawyer in California sued two crypto companies, MoonPay and Yuga Labs, accusing them of using a “vast network of A-list musicians, athletes and celebrity clients” to mislead investors about digital assets.

In March, the S.E.C. charged the actress Lindsay Lohan, the online influencer Jake Paul and musicians including Soulja Boy and Lil Yachty with illegally promoting crypto assets. And in late May, after months of failed attempts, a process server delivered court papers to Shaquille O’Neal, the retired basketball star, who was sued for promoting FTX, according to legal filings. Mr. O’Neal was served while broadcasting from a National Basketball Association playoff game.

Representatives for Mr. Brady, Mr. Bankman-Fried and MoonPay declined to comment. A spokeswoman for Yuga Labs said the company had “never paid a celebrity to join the club.” Representatives for Ms. Bündchen and Mr. O’Neal did not respond to requests for comment.

Tech start-ups and celebrities have long had a symbiotic relationship. The start-ups offer stars a way to make money while staying on the cutting edge of internet culture; the celebrities help young companies gain credibility and reach a larger audience.

Of all the start-ups that recruited celebrities to endorse crypto, FTX was perhaps the most eager. As Mr. Bankman-Fried tried to turn FTX into a household name, he made a list of celebrities he could envision promoting the company, recalled Mr. Nader, the former FTX executive. Mr. Brady’s name was at the top.

A former college football player, Mr. Nader was in charge of recruiting Mr. Brady and other stars. In June 2021, Mr. Brady and Ms. Bündchen agreed to a deal with Mr. Bankman-Fried, praising the “revolutionary FTX team.” Mr. Brady seemed genuinely interested in crypto, Mr. Nader said, and occasionally had conversations with Mr. Bankman-Fried.

“Imagine a tiger and a lion talking,” Mr. Nader said. “They’re slightly different, they do different things, but they’re really formidable in their own arenas.”

In 2021, Mr. Brady also co-founded Autograph, which helps famous people sell the crypto collectibles known as nonfungible tokens, or NFTs. Autograph raised more than $200 million from investors, and Mr. Bankman-Fried joined the board.

That same year, Mr. Brady and Ms. Bündchen starred in a $20 million advertising campaign for FTX, with commercials that ran during N.F.L. games. Mr. Brady also posted TikTok videos with Mr. Bankman-Fried from FTX’s headquarters in the Bahamas, where he spoke at a conference in front of hundreds. Backstage, Mr. Bankman-Fried remarked that he could imagine buying a football team someday with Mr. Brady. Ms. Bündchen also appeared at the conference as FTX’s head of environmental and social initiatives.

When FTX collapsed last November, the company’s $32 billion valuation — including Mr. Brady and Ms. Bündchen’s $48 million of shares — plummeted to zero. The couple had also received a small amount of Ethereum, Bitcoin and Solana tokens to trade on the platform, one of the people said, which disappeared in FTX’s bankruptcy.

Mr. Brady has not commented publicly on FTX or his relationship with Mr. Bankman-Fried. After FTX’s crisis meeting in November, Mr. Nader called him back.

“He was concerned,” Mr. Nader said. “The very first thing he asked me was: ‘Sina, how are you doing? I know you put your heart and soul into this.’”

Ms. Bündchen said in a March interview with Vanity Fair that she had “trusted the hype” and felt “blindsided.”

Mr. Brady’s other crypto venture has also struggled. Autograph’s revenue sank last year amid the crypto meltdown, a person familiar with its finances said. The start-up has shifted its strategy to focus more on helping celebrities find ways to foster loyalty with their fans, and less on marketing crypto tokens to consumers, the person said. The firm has also removed some crypto language from its marketing, downplaying terms like NFT, another person with knowledge of the company said.

Autograph has also cut more than 50 employees in layoff rounds, a third person said. The reductions were reported earlier by Insider. An Autograph spokeswoman declined to comment.

Mr. Brady has also faced legal trouble. In December, Adam Moskowitz and the law firm Boies Schiller Flexner filed a lawsuit in federal court in Florida accusing him and Ms. Bündchen of misleading investors. Among the other defendants are the comedian Larry David, the N.B.A. star Steph Curry and the tennis player Naomi Osaka, all of whom endorsed FTX.

“None of these defendants performed any due diligence prior to marketing these FTX products to the public,” the lawsuit said.

Some celebrities narrowly escaped the crypto mess. Katy Perry, the pop star, held talks about a partnership with FTX that never came to fruition, three people familiar with the situation have said.

In spring last year, Taylor Swift discussed a deal with FTX that could have paid as much as $100 million, two people familiar with the matter said. A tour sponsorship was on the table after Ms. Swift declined other promotional options, a person with knowledge of the talks said. The deal’s size was reported earlier by The Financial Times.

Mr. Moskowitz, the lawyer suing the celebrities, said on a podcast in April that Ms. Swift had conducted due diligence on FTX, asking the exchange to prove that its cryptocurrencies were not unregistered securities. His comments led to a flurry of headlines about Ms. Swift’s business acumen. But in an interview with The New York Times, Mr. Moskowitz said he had no inside information about the talks.

In reality, Ms. Swift’s side signed the sponsorship agreement with FTX after more than six months of discussions, three people with knowledge of the deal said, and it was Mr. Bankman-Fried who pulled out. The last-minute reversal left Ms. Swift’s team frustrated and disappointed, two of the people said.

A spokeswoman for Ms. Swift declined to comment.