A way to continue a mailing that was cancelled?
This is not pretty, but you can use the API from your browser to un-schedule the mailing.


Find the ID of your mailing


Go to Mailings > Scheduled and Sent Mailings
Find your scheduled or canceled mailing within the list
Determine the ID of the mailing by hovering over Report and looking for the number after mid= in the URL

Open a javascript console in your web browser


Begin with your web browser on any CiviCRM page.
In Firefox go to menu > "Developer" > "Web Console". In Chrome, go to menu > "More tools" > "Developer Tools" > "Console".
If you can type 2+2, press Enter, and see 4, you're all set.

Execute some API commands to un-schedule the mailing


Copy-paste the following code into the console (but don't press Enter yet)

var mailingID = 0;
CRM.api3('MailingJob', 'get', {'mailing_id': mailingID}).done(
  function(response) {
    CRM.api3('MailingJob', 'delete', {'id': response.id});
  }
);
CRM.api3('Mailing', 'create',
  {'id': mailingID, 'scheduled_id': '', 'scheduled_date': ''});

Update the number 0 after mailingID to your own ID value determined above.
Press Enter to execute the commands.

See the results


Go to Mailings > Draft and Unscheduled Mailings and find your mailing here, waiting for you to Continue