Follow the email sending checklist to ensure all settings are in place before you hit the send button.

You can unschedule previously scheduled messages any time before the scheduled date/time. Open the scheduled message and click the "Unschedule" link in the message editor.

It's not possible to stop messages once email sending starts and the email delivery is in progress

You can resend a message to a single contact after the blast has been sent. Open the sent message and click the "Send to single contact" button then type the email address of the contact you wish to re-send the message to. If the contact doesn't show up on the list they are either unsubscribed or don't exist in the databased.

The "Send to single contact" option is not available for messages that used AB testing.

You can resend a previously sent message to contacts who didn't open or click the original email. This article explains how.