Manually started scheduled email not completing
Do you have a cron job configured for CiviCRM that runs periodically? 

Can you check CiviCRM and/or Apache Error log for any errors?

Scheduled Mailing jobs are executed in batches of 1000 or the batch limit specified under Administer >> CiviMail >> Mailer Settings. 

If the recent Schedule job log says 'Started' then the last job was terminated because of error. This might be for several reasons like php time out, memory allocation, mysql connection time out, DB errors etc. To trace the problem you will need to check CiviCRM and Apache logs to see if any errors were recorded. 

To complete the job you can re run the Send Scheduled Mailings manually if your system is not configured to do it automatically.