It was already a tough day in the market when the unexpected hit.
The New York Stock Exchange halted trading late Wednesday morning because of technical trouble.
The nearly four-hour outage came as traders had plenty of other things to worry about. Concerns about China's plunging stock market and a logjam in talks between Greece and its creditors weighed on the mood. Major indexes were already falling before the shutdown, which occurred shortly after 11:30 a.m. Eastern time. NYSE resumed trading at 3:10 p.m.
The exchange didn't say what caused the malfunction but described it as internal and not the result of hackers.
The broader stock market stayed open as orders to buy and sell kept flowing to the Nasdaq and other exchanges around the country.
Tom Caldwell, who runs an investment firm with stakes in several exchanges, said there are some 60 exchanges and trading venues that can take orders when one goes down, so investors shouldn't get rattled.
"It's disruptive, but not wildly disruptive," said Caldwell, chairman of Caldwell Securities.
President Obama was briefed on NYSE and United's problems, according to Josh Earnest, the White House spokesman. "There is no indication at this point either that there is malicious activity involved or that it was related to any of the other high-profile technology issues that have cropped up today," Earnest said.
On Tuesday, Anonymous, the infamous hacker group, tweeted a statement about potential problems with Wall Street.
U.S. markets have been dogged by technical problems over recent years as more trading is handled by computers. In May 2010, the Dow plunged hundreds of points in minutes in an incident that later became known as the "flash crash." In March 2012, BATS Global Markets, a Kansas company that offers stock trading services, canceled its own IPO after several snafus.
Two months later, a highly anticipated IPO of Facebook on the Nasdaq exchange was marred by a series of technical problems, rattling investors unsure if their orders went through.
James Angel, an associate professor of finance at Georgetown University's McDonough School of Business, said NYSE's shutdown highlighted both the fragility and the resilience of modern technology. Angel sat on the board of exchange company Direct Edge before it was acquired by a larger rival last year.
"From an investors' perspective, if you hadn't heard about the outage, you probably wouldn't have noticed," Angel said.
Portfolio manager Mark Spellman of Alpine Funds said an outage similar to Wednesday's would have caused panic a few decades ago, when the NYSE dominated the market. But firms making trades were able to use a variety of other exchanges while the NYSE was out of commission. He says the disruption didn't cause any problems for the global markets.
"Only 15 to 20 percent of global stock exchange trading happens on the NYSE these days," he said. "Things are so spread out."
Still, others on Wall Street found the long outage unsettling.
Phil Orlando, chief equity strategist at Federated Investors, said it was unsettling that computer problems also forced United Airlines to temporarily ground its flights across the country and the Wall Street Journal's website went down, all on the same day.
"These are visible icons of American industry," he said. "It's just unnerving."
In China, the Shanghai Composite sank 6 percent despite new attempts by China's government to stop the selling. Hong Kong's Hang Seng, a victim of the turmoil in mainland Chinese markets, also lost 6 percent. Beijing ordered state-owned companies to buy shares and promised more credit to finance trading. The Shanghai index has lost almost a third of its value in the last month. It is still up 70 percent over the past year.
In Europe, Greece applied for a new three-year loan and said it would have a new proposal for creditors in coming days. The deeply indebted country needs a financial lifeline from its European lenders before its banks collapse, an event that could push Greece out of the currency union.