How do I send a scheduled mailing via CiviMail?
Mailings run from a cron job on the server, so submitting immediately does not immediately start the mail sending,  it sends the next time the bulk mail processor task runs.

If the mailing is still not sending in an hour or so...

Go to Scheduled jobs (Administer>System Settings> Schduled Jobs)
And check that the Mailings scheduler job is enabled.

If it is enabled, and your mailing still hasn't gone talk to your admin person to check that cron jobs are enabled on the server.

Details fpor configuring scheduled jobs here: https://docs.civicrm.org/user/en/latest/initial-set-up/scheduled-jobs/

Cheers

Craig