CiviMail Mailing Scheduled But Not Sent
It's possible that your cron isn't set up.  I would check to make sure that your Status Console (Administer menu » Administration Console » System Status) doesn't have a "Cron Not Running" message (check the hidden statuses if there are any!).

It's also possible that the Scheduled Job to send mailings hasn't been enabled. Check Administer menu » System Settings » Scheduled Jobs and ensure that Send Scheduled Mailings is enabled.  You can also see the last time it ran and ensure it matches your expectation from cron.

Finally, if the job DID run, click the View Job Log link next to the Send Scheduled Jobs record.  If you're not sure how to interpret the result, please edit your original question to include the log.