As U.S. markets opened Monday morning, 30-year old engineer Vivek Sevak was ready to cut his losses.

The recreational trader had put money in a market-short exchange traded fund, and was looking to sell. But when he logged into his Robinhood account to make the trade, he found an error message. The stock-trading platform was down.

"It's frustrating. You're just sitting there, stuck on the sidelines," Detroit-based Sevak told CNBC in a phone interview. "I had to watch those ETF prices go down and just eat the losses."

Sevak was one of potentially millions unable to execute trades on the Robinhood app on Monday. The online brokerage firm said it experienced a "system-wide outage" that lasted through the close. Meanwhile, the Dow Jones Industrial Average posted its biggest percentage gain in more than a decade.

"We are working to resolve this issue as soon as possible," the company said in a message to clients Monday. With an hour left in the trading day, Robinhood said the issue had been "identified and a fix is being implemented." As of 10 p.m. ET Monday, the platform was still down.

Sevak was not alone. Many of Robinhood's 10 million users took to Twitter and Reddit to blast the start-up. Some said they would leave the platform, while others threatened a class-action lawsuit.

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Legally, Robinhood may have protected itself through a 44-page customer agreement form. By clicking "I agree" when signing up for the app, users consent to these potential losses — even if they don't read past the first line.

"For the customers, it's most likely tough luck," said James Angel, associate professor at Georgetown's McDonough School of Business. "This won't stop the class-action lawyers from launching nuisance lawsuits, however."

On the fifteenth page of the agreement, Robinhood outlines that it will not be responsible for "temporary interruptions in service due to maintenance, Website or App changes, or failures" and isn't liable for extended interruptions due to failures "beyond" the company's control. That includes computer viruses, "forces of nature," labor disputes and "armed conflicts."

"Robinhood does not warrant that these channels will be available and error free every minute of the day," according to the document.