Slack, the popular messaging platform used by millions of people worldwide, restored service after experiencing a major disruption Monday as many U.S. employees returned to work after the holidays.

The company initially called the service problem an "incident" in a statement on its website Monday morning, then upgraded it to an outage an hour later.

"Customers may have trouble loading channels or connecting to Slack at this time," the statement said. "Our team is investigating and we will follow up with more information as soon as we have it."

At 3:10 p.m., the company wrote: "We're truly sorry for the disruption today. We'll be back with a summary of this issue."

A Slack representative said the company was "seeing improvements with error rates on our side, and we believe affected customers should be able to access Slack."

The company said it was continuing to work to resolve issues with its calendar apps and e-mail notifications.

The representative could not say how many of Slack's more than 10 million daily users were affected or what had caused the disruption.

The website Downdetector, which tracks internet disruptions, recorded a spike in reported problems with Slack about 10 a.m. Eastern time, and the company posted its first statement about the problem at 10:14 a.m.

During the outage, users could not send messages, load channels, make calls or log in to the service, and some had problems with their calendars and notifications. The outage sent many people either to Slack's competitors, like Google or Zoom's video services, or back to phone calls and e-mails — tools that could feel antiquated to some employees who, while working from home for much of the last year, have grown used to the convenience and immediacy of the Slack app on their laptop or smartphone.

Service began to resume for some users around 12:20 p.m. Eastern. "Some customers may be able to connect, but may also experience degraded performance," Slack said at 1 p.m. ET.

As it has grown in recent years, Slack has become an essential workplace tool, with many users in media organizations and companies that have shifted to working from home because of the coronavirus pandemic. More than 750,000 companies use the service, according to Slack.

Salesforce, a company that sells marketing and sales software, announced in December that it would buy Slack for $27.7 billion in cash and stock, the latest in a series of major deals showing the demand for tools that allow people to work remotely.

In September, Google services, including Gmail, Hangouts, Maps and YouTube, briefly crashed, as did Slack and Microsoft's Outlook, Office 365 and Teams. In December, Google suffered another outage with its apps that lasted about 45 minutes.