https://www.theregister.co.uk/2019/11/21/british_airways_flight_delays_it_problem/

British Airways suffered yet another IT failure, causing knock-on flight delays and cancellations for thousands of unhappy travellers*.

An unspecified computer system crash has meant some BA's fleet has been grounded for hours and counting - it seems to be largely an inbound flight issue. Long-haul flights appear to be have been the worst hit, giving a hint about the cause of the problem.