It's not certain what triggered the outage, but BA says it hasn't seen evidence of an attack on its network.
To say this is causing chaos would be an understatement. It's a bank holiday weekend in the UK, so the terminals were chock full of travelers hoping to visit friends and family -- the cancellation likely just wrecked their plans. While many of them will get compensation, that's not much comfort given the timing.
And this is a dire situation for British Airways, of course. It's the sixth major crash for its recently installed system in the space of roughly a year, and the timing makes it one of the highest-profile incidents to date. It suggests there's something flawed with its newer infrastructure, and that there's no simple fix. It also underscores the importance of reliability for transportation tech. Even a relatively rare crash bug can become a serious headache if it tarnishes an airline's reputation and results in expensive payouts to angry travelers.
Update: British Airways chief Alex Cruz has explained in a video apology (below) that the failure was likely caused by a "power supply issue." The airline is promising a full refund if you were affected and no longer want to fly (or just can't), so there's some consolation here. Still, the explanation makes the situation all the more outlandish: all it took was a power management problem to throw one of the world's largest airlines (not to mention largest airports) into chaos.