Mass IT outages have caused disruptions around the world – with thousands of flights canceled or delayed and online services down at airports, hospitals, banks and businesses. Many Microsoft users started their day with an error screen – dubbed the “blue screen of death,” or BSOD error.

The company says it is aware of an issue affecting Windows programs running technology from CrowdStrike, a cybersecurity company. Here’s what to know about the widespread IT outages and what happens next.

Where have users reported outages?

At 9 a.m. Eastern time, more than 2,100 flights had been canceled worldwide. More than 1,200 of those were flights operating into, within or out of the United States, according to FlightAware.com, an online flight tracking website. More than 22,000 flights were delayed worldwide, with delays affecting more than 2,600 flights originating and/or ending in the United States.

Major U.S. carriers including United Airlines, American Airlines and Delta Air Lines grounded their flights overnight, as the Federal Aviation Administration cited communications issues. Airline operations are beginning to resume some flights, but, as United later said, disruptions are expected “to continue throughout Friday.” United blamed a “third-party software outage” that impacted computer systems worldwide. American Airlines also pointed to a “technical issue with a vendor” that disrupted operations.

The Department of Homeland Security said it is “working with CrowdStrike, Microsoft and our federal, state, local and critical infrastructure partners to fully assess and address system outages.”

Airports in the Netherlands, Germany, Britain, Israel, Hong Kong and South Korea were among those affected. Some staffers turned to manually checking in passengers as a result of the digital disruption.

What is the cause of the Windows outage, and when was it reported?

Details about the outage, which has also affected websites, broadcasters and payment systems, are still emerging. According to Microsoft Azure, a cloud computing platform, issues were first reported around midafternoon Thursday.

Microsoft said the “preliminary root cause” appeared to be “a configuration change in a portion of our Azure backend workloads.” In turn, it said, that “caused interruption between storage and compute resources which resulted in connectivity failures that affected downstream Microsoft 365 services dependent on these connections.”

Early Friday, Microsoft said it had “been made aware of an issue” affecting Windows users running technology from CrowdStrike, a cybersecurity company, called the Falcon agent. Falcon, according to the company website, is a platform to stop online security breaches via cloud technology. It encompasses more than 10 different security and IT tools including AI technology.

Microsoft said that users “may encounter a bug check” – or the “blue screen of death” – “and get stuck in a restarting state.”

George Kurtz, the chief executive of CrowdStrike, said early Friday in a post on X that “the issue has been identified, isolated and a fix has been deployed.” He urged customers to utilize the company’s support portal for the latest updates.

CrowdStrike was set up in 2011 by co-founders Kurtz and Dmitri Alperovitch to provide security for the world’s leading businesses using “endpoint protection” and “expert intelligence to pinpoint … adversaries” who are launching cyberattacks, according to its official website.

What have Microsoft and CrowdStrike said about the outages?

Kurtz blamed “a defect … in a single content update for Windows hosts.” Mac and Linux hosts were not impacted, and “this is not a security incident or cyberattack,” he added in a post on X.

In a later appearance Friday morning on NBC News’s “Today” show, Kurtz said the company was “deeply sorry for the impact that we’ve caused to customers, to travelers, to anyone affected by this.” Many CrowdStrike systems are recovering and will be soon be operational, Kurtz said, but “it could take some time for some systems that won’t automatically recover.”

In a separate update, Microsoft said some customers have managed to resolve the issue by “attempting multiple Virtual Machine restart operations,” although it appeared that “several reboots” – as many as 15 – could be required.

It also noted that users “may be unable to access various Microsoft 365 apps and services,” including Microsoft Teams and the Microsoft 365 admin center. “The underlying cause has been fixed, however, residual impact is continuing to affect some Microsoft 365 apps and services,” it said on X.

What services have been affected by the mass IT outages?

Health care: A number of hospitals and health services around the world have been disrupted by the outage. In the United States, the Memorial Sloan Kettering Cancer Center in New York said Friday that it is “pausing the start of any procedure that requires anesthesia,” due to a “worldwide technical disruption.” Massachusetts General Hospital said that all previously scheduled non-urgent surgeries, procedures, and medical visits would be canceled “due to the severity” of the outage.

England’s National Health Service said the outage disrupted “the majority” of family practice surgeries on Friday, affecting digital appointment and patient record systems. It said paper patient records would be used and that patients should continue to attend appointments.

Israel’s Health Ministry said the outage has hit more than a dozen hospitals and that many are operating on a manual basis while their digital services remain inaccessible. Two hospitals in the northern German cities of Luebeck and Kiel have also canceled elective surgeries scheduled for Friday, Reuters reported.

Emergency phone lines: Alaska State Troopers said early Friday that “many 911 and non-emergency call centers are not working correctly” across the state, citing a “technology-related outage.” It redirected emergency callers to different numbers in a Facebook post.

Paris 2024 Olympics: The Paris Olympics organizing committee said Friday that some of its IT services have been disrupted by “the global technical issues affecting Microsoft software,” though it stressed that ticketing systems were unaffected and “the preparation of venues is continuing normally.” The outage has had a “limited” impact so far, affecting “the delivery of uniforms and accreditations,” while some delegations were experiencing flight delays, it added. The Olympics are due to start next week.