If we lose the entire cluster, we can't do anything other than get it back up and running as soon as possible. It's important to have monitoring, and to put a proper process in place to understand what needs to be done, when, and by whom, should this ever happen.

Recovering when the entire cluster goes down essentially involves restoring from backups and setting up new shards, which is complicated and will take time. Dry testing this in a staging environment is also advisable, as  is investing in regular backups via MongoDB Ops Manager or any other backup solution.