“On Monday, we experienced an issue with a part of our infrastructure that resulted in an outage, preventing customers from using our app, website, and help center,” a Robinhood spokesperson said in a statement provided to Fortune. “We realize we let our customers down, and we’re committed to improving their experience.”

Service at the stock trading app was only restored early Tuesday morning, but even then Robinhood struggled to keep it online. By mid-morning, a company Twitter account stated the service had gone down anew:

In its statement to Fortune sent early Tuesday morning, Robinhood said it would reach out to customers affected by the outage, and work with them on a case-by-case to consider “billing credits and/or other potential compensation.”

The spokesperson also urged users to check for updates on a status page (status.robinhood.com) that on Monday described a “System-Wide Outage” across all of Robinhood’s apps, trading and banking services. A screenshot from Monday reflects the scale of the outage:

Missing the entire stock trading day is an unprecedented situation for the company, and one almost unheard of in the brokerage industry, where outages that last minutes are enough to cause a hubbub.

Early on Tuesday morning, a Robinhood spokesperson described the cause of the outage as “instability in a part of our infrastructure that allows our systems to communicate with each other,” and stated that it was not caused by a failure to code for Leap Year.

A further possible cause of the Robinhood outage is a surge in volume spurred by investors spooked by gyrations in the market related to news of the coronavirus.

According to an executive from Apptopia, a service that analyzes apps, the number of active daily Robinhood users has been “skyrocketing” since early January. The executive added it’s unclear if most of these Robinhood account holders are buying or selling.

Robinhood’s outage comes at an awkward time for the company, which had been distancing itself from several controversies last year, and recently touted growth that saw the app top more than 10 million users.

Meanwhile, the startup must persuade investors it can grow into a $7.6 billion valuation, even as traditional brokerage rivals have started to match its zero-fee commission model.

The goodwill Robinhood enjoys with millennial customers, who have flocked to its sleek design and feel-good ethos, is likely to be tested by the stock-trading app going down, which has led to users besieging Robinhood’s Twitter accounts with complaints.

Numerous users have also complained on Twitter that Robinhood outage had hurt their ability to profit from trading, with some suggesting they would take legal action.

This story was updated to reflect comments provided by Robinhood at 3:45am ET on Tuesday.

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