New York (CNN Business) Robinhood, the free stock-trading app, crashed on the worst day possible.

Stocks rallied Monday with the Dow logging its biggest point gain in history . The S&P 500 gained $1.1 trillion Monday. But an all-day system-wide outage left Robinhood customers seething.

An "issue with a part of our infrastructure" caused the 17-hour disruption on Monday, the company said in a statement. Robinhood said overnight that the issue was fixed, but the service went down again Tuesday morning shortly after trading began.

Around noon Tuesday, Robinhood said the earlier outage had been resolved.

Robinhood, which has about 10 million users, told customers in an email that the funds in their accounts are safe and personal information was not breached. Despite those reassurances, people slammed the app on Twitter. Some called the outage "ridiculous" and other demanded compensation.

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