Townlink's license to operate has been revoked with effect from 26th February but they've chosen to leave passengers in the lurch and have already stopped running the 21.
But Ensign Bus have come to the rescue and are running a replacement 21 service between Brentwood and Ongar on weekdays only. Here's the timetable:
Updated timetable from 20th January:
Other buses are available for journeys beyond Ongar. See Traveline to plan a journey.