The New York Stock Exchange is back in business, but investors are still feeling a little queasy about Greece and China.
NYSE enjoyed a glitch-free day on Thursday following a major technical problem that shut down trading for nearly four hours on Wednesday.
The Dow ended up 33 points on the day, but that represents a disappointment considering it was nearly 250 points in the green shortly after the opening bell. There didn't appear to be any major drivers for the rally to fizzle other than a lack of enthusiasm and global uncertainty.
The back-and-forth action represents just the latest turbulence to hit U.S. stocks amid concerns about the lingering crisis in Greece and scary stock market crash in China.
"It's been a rollercoaster. We're violently lurching around in a box here. It's like an angry Siamese cat in a cardboard box," said Michael Block, chief strategist at Rhino Trading.
Still, the return to normalcy at NYSE should allow investors and regulators to breathe a sigh of relief. Wednesday's trading halt generated international headlines and briefly raised concerns about a cyber attack. However, NYSE and regulators say the outage was caused by a botched software update, not hackers. There weren't any major problems reported on Thursday.
"Business as usual. Whatever ghost was in the machine yesterday has obviously been extracted out," Keith Bliss, director of sales and marketing at Cuttone & Company told CNN from the trading floor.
Stock pop: Stocks surged right out of the gate on Thursday, joining in a global rally that started in the rollercoaster Chinese stock market.
Related: What the heck happened at NYSE?! CNNMoney answers your questions
The S&P 500 and the Nasdaq advanced about 0.25% a piece. The gains bring the S&P 500 back to breakeven on the year, although the Dow remains a bit in the red.
Wall Street cheered a huge rebound in China, which is scrambling to put a bottom under its crashing stock market. The Shanghai Composite declined another 3% at the opening bell and then did a 180 and ended the day up a whopping 5.8%
What caused the outage? NYSE believes the glitch was caused by a problem with a software update it rolled out on Tuesday evening, the exchange said in a new statement released on Thursday.
NYSE said the new software caused a communication problem on Wednesday morning between traders and the exchange. An initial fix allowed stocks to open as scheduled but additional communication issues arose by mid-morning. That's when NYSE decided to pull the plug and suspend trading -- a shutdown that lasted nearly four hours.
Related: Stock market crashes: How does China's stack up?
It's important to note that investors were still able to buy and sell stocks despite the glitch. The outage only impacted NYSE, leaving other exchanges like Nasdaq (NDAQ) and BATS to pick up the slack.
As a result of the trading halt, NYSE's share of the trading action tumbled to a record low on Wednesday, according to Nanex, a firm that studies stock market data. Trading resumed shortly after 3 pm ET.
The problem highlights the complex nature of the trading systems and software at the heart of the stock market.
The NYSE outage was hardly the first glitch to impact the U.S. stock market. Previous problems include the 2013 incident at the Nasdaq that halted trading on all Nasdaq-listed stocks and the 2010 Flash Crash that mysteriously sent U.S. stocks plummeting before a rebound.
