My Magento cron is not running at all, how do you fix it?

First we check the crontab for the line * * * * * php bin/magento cron:run (and the matching setup:cron:run). If it is missing we run bin/magento cron:install and confirm the user, PHP binary and path are correct. Then we watch the cron_schedule table to confirm jobs move from pending to success. A dead cron is the most common reason emails, the sitemap and indexers all "stop working" at once.