I set up and scheduled a mailing, but it didn't send. Now what?
This has happened to me in the past for a few reasons


In one instance, my time zone was set wrong so the scheduled time had already passed, and the mailing was never triggered.
My cron jobs weren't running. A temporary fix would be to trigger the job manually in Administer > System Settings > Scheduled Jobs > Send Scheduled Mailings.
My cron job hadn't run yet. For example, I can schedule my email to go out at 1:15, but if my cron jobs run every hour it won't go out until 2:00.