Airlines, banks, TV channels and other business across the globe were scrambling to deal with one of the biggest IT crashes in recent years on Friday, apparently caused by an update to an antivirus program.
Major US air airlines initially grounded all flights over a communication issue -- though American Airlines later said it had reinstated its flights.
Airports across the world said check-in systems were down and services were being handled manually, with delays likely.
Microsoft said in a technical update on its website that the problems began at 1900 GMT on Thursday, affecting users of its Azure cloud platform running cybersecurity software CrowdStrike Falcon.
"We recommend customers that are able to, to restore from a backup from before this time," the US software giant said.
CrowdStrike CEO George Kurtz said on social media platform X that customers had been "impacted by a defect found in a single content update for Windows hosts".
"The issue has been identified, isolated and a fix has been deployed," he said.
Shares in CrowdStrike slumped by 20 percent in pre-market trading.
From Amsterdam to Zurich, across all continents, airports were reporting problems with their check-in systems.
"I'm just in limbo as to how long I've got to wait here," flight passenger Alexander Ropicano told AFP, as he waited at Sydney Airport in Australia.
The 24-year-old, flying to Brisbane to see his girlfriend, said the airline told him to "wait until the system is operational again", but there is no indication how long that could be.
Media companies were also struggling with Britain's Sky News saying the glitch had ended its morning news broadcasts and Australia's ABC similarly reporting a major "outage".
Some banks reported difficulties in processing digital payments, mobile phone carriers were disrupted and customer services in a number of companies were down.
The global nature of the failure prompted some experts to call for greater resilience in networks and question the reliance on a single provider for such a variety of services.
"We need to be aware that such software can be a common cause of failure for multiple systems at the same time," said software engineering professor John McDermid from Britain's York University.
"We need to design infrastructure to be resilient against such common cause problems," he added.
Airports and airlines were the most dramatically affected, with US airlines grounded early on Friday.
All flights "regardless of destination" were grounded because of a "communication issues", the FAA said in a notice to airlines.
However, American Airlines later said that as of 0900 GMT "we have been able to safely re-establish our operation".
"We apologise to our customers for the inconvenience," the airline said.
Major airports including Berlin, which had earlier said all flights had been suspended, said flights were gradually resuming after the "technical issue".
All airports in Spain were experiencing "disruptions" from an IT outage, the airport operator Aena said.
Hong Kong's airport also said some airlines had been affected, with its authority issuing a statement in which it linked the disruption to a Microsoft outage.
The UK's biggest rail operator warned of possible train cancellations due to IT issues.
Australia's National Cyber Security Coordinator said the "large-scale technical outage" was caused by an issue with a "third-party software platform".
France's cybersecurity agency ANSSI said it was "fully mobilised" to identify and support those affected.
"There is no evidence to suggest that this outage is the result of a cyberattack," the agency said.
