According to TfL, there is a significant outage with the Countdown service since this morning (
@TfL have been replying to queries all day on Twitter). Apparently, there was a power cut affecting one of the data centres that delivers the Countdown service. I'm seeing some live departures on Citymapper, but it's clearly still affecting other services, including
https://tfl.gov.uk/modes/buses/live-bus-arrivals, so possibly they're currently trying to bring it back online?