Slack, the messaging services used by millions of people for work and school, has suffered a global outage on the first day back for most people returning from the New Year's holiday.
It's the latest tech glitch to show how disruptive technical difficulties can be when millions of people are depending on just a few services to work and go to school from home during the pandemic.
The company stopped releasing its daily user count after topping 12 million last year.
"Our team is currently investigating and we're sorry for any troubles this may be causing," Slack said in a prepared statement.
The outage began about 10am US eastern time on Monday (2am AEDT on Tuesday) and disrupted service in the US, Germany, India, the UK, Japan and elsewhere.
At 4.30am AEDT, service was still sporadic and Slack said the outage was ongoing but that some users may begin to see improvement.
Slack said that people should check status.slack.com for updates.
The outage comes about a month after Salesforce.com said it would acquire Slack for $US27.7 billion ($A36.1 billion).
The deal is aimed at giving the two companies a better shot at competing against long-time industry powerhouse Microsoft.