BA suffered a massive fault last week when it suffered a power surge following an employee disconnecting the supply. Upon reconnection, there was a surge that knocked out the system and resulted in all flights from Gatwick and Heathrow being cancelled on Saturday. According to the BBC the power shutdown somehow managed to bypass the battery backup systems known as uninterruptible power supplies (UPSs).
What's not clear is how a power surge at one was able to take down the two sites that BA uses. They're separated by a kilometre and are almost certainly powered entirely independently of each other.
The Register reports that it may have been an "uncontrolled restoration of power" which resulted in the surge. BA said that the person responsible was authorised to be in the data centre but not to be working on the power. This makes sense in the context of the report that the power restoration was handled badly, as there will almost certainly be clear processes to follow for restoring power.
Turning back on an entire data centre in one go is probably a bit like trying to boil 100 kettles at home during the Corrie ad break. It's inadvisable and likely to trip a switch at best and set fire to everything at worst.
BA has appointed an independent firm to investigate the failure and produce a report.