report mailing date and time have expired
If your cron is not running then there is nothing to 'tell' the Scheduled Jobs (which starts the mailing job) to run.

As a quick fix you should be able to go to your Scheduled Jobs (Admin > System > Scheduled Jobs, or /civicrm/admin/job?reset=1 in Drupal), find the Mailings scheduler job and use 'Execute Now' under the 'More' option - this should cause any overdue Mailings to be sent out.

To set up a cron so that the Jobs run automatically you need to follow the instructions here: https://docs.civicrm.org/sysadmin/en/latest/setup/jobs/

You also need to set any relevant Scheduled Jobs to be 'enabled' (under More)