"The problems have emerged across the world, but was first noticed in Australia, and possibly felt most severely in the air travel industry.
There are delays at UK airports, with long queues reported at London's Stansted and Gatwick.
Ryanair said it had been "forced to cancel a small number of flights today (19 July)" and advised passengers to log-on to their Ryanair account, once it was back online, to see what their options are.
British Airways also cancelled several flights.
Several US airlines, notably United, Delta and American Airlines, grounded their flights around the globe. Australian carriers Virgin Australia and Jetstar also had to delay or cancel flights as departure screens went blank at Sydney airport.
Airports in Tokyo, Amsterdam and Delhi also said services had been affected.
Meanwhile, the problems have also hit trains, payment systems and healthcare providers.
UK railway companies reported delays and said they were experiencing "widespread IT issues".
Payment systems - many shops could only take cash. In the UK, supermarkets including Morrisons and Waitrose spent the morning unable to accept contactless payments. It was the same in Australian supermarkets including Woolworths and Coles, while financial institutions such as the National Australia Bank were also affected.
Healthcare - Israel said 15 hospitals had switched to manual processes, although this did not affect medical treatment. Ambulances were told to take new cases to other hospitals. In the UK, some doctors' surgeries in England reported issues with booking appointments.
As the full extent of the disruption became clear, more firms and institutions started reporting problems.
The US state of Alaska warned that its emergency services were affected.
Broadcasters were also caught up in the chaos, including Sky News in the UK, which spent several hours off air.
The London Stock Exchange said it was working as normal, but there were issues with its news service, used by companies to report market-sensitive information in a timely way.
And Poland’s largest container terminal, the Baltic Hub in the northern city of Gdansk, said the outage was "hampering terminal operations" and asked companies not to send containers to the port."