Robinhood, the trading app popular with millennials, experienced a major outage on Monday that locked out iOS, Android, and web users for nearly the entire day of trading.

The outage came just as the market rebounded from one of the worst weeks since the financial crisis.

Robinhood users are furious – and some are demanding the company reimburse them for the day of missed trading.

Robinhood emailed users at close saying it’s “working as hard as we can to resume service.”

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A service outage took Robinhood offline for nearly the entire day of trading on Monday – and furious users of the commission-free trading app say it couldn’t have come at a worse time.

People were locked out of Robinhood’s iOS, Android, and web apps from Monday morning until trading closed in the afternoon, according to Robinhood’s status site.

The company said it began investigating a “system-wide outage” shortly after 9:30 a.m. ET. By 11:30 a.m., Robinhood said that it had identified the problem and that “a fix is being implemented,” but all its trading services were still experiencing major outages by the time trading closed at 4 p.m.

Robinhood sent an email to its users at close stating that it was “working as hard as we can to resume service.”

While Robinhood was offline, the market was rebounding from one of the worst weeks since the 2008 financial crisis. The Dow Jones industrial average rose over 1,294 points on Monday – its biggest point gain ever – after falling by more than 3,500 points last week as fears about the coronavirus outbreak swirled.

People on Twitter and the Robinhood subreddit voiced frustration and anger as they watched the market climb without being able to access Robinhood trading on Monday.

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This is outrageous, I've watch over 33% of my investments disappear without being able to do anything, because you were negligent with your servers! — Tom Kofer (@RealBunnyColvin) March 2, 2020

By Monday afternoon, a Twitter account called “Robinhood Class Action” had been created and gained over 1,500 followers. The account’s bio said it was “building a case against Robinhood for their negligence and late open on March 2.”

Robinhood has costed investors BILLIONS for the late open this morning. Reply with your story and exactly how long it took to get the app to trade function. #robinhood #robinhoodapp — Robinhood Class Action (@ClassRobinhood) March 2, 2020

Robinhood did not immediately respond to Business Insider’s request for comment.

The company has run afoul of federal authorities in the past – in December it was hit with $1.25 million in fines by the Financial Industry Regulatory Authority over concerns that it didn’t give its users the best deals on their trades.