Post Mortem Analysis: PostgreSQL replication failed
Based on this I'd say you didn't have enough wal_keep_segments on the master, weren't using a replication slot, and either had hot_standby_feedback off or had the connection drop for long enough for the master to remove needed WAL.

And you are presumably not using WAL archiving (archive_command on master, restore_command on replica) as fallback.

So the master removed transaction logs the standby needed.

You'll need to re-create the standby. Then either:


Set the standby to use a replication slot and enable hot_standby_feedback; or
enable archive_command and restore_command