Select the list where the scheduled after opt-in message is located by clicking the Manage button from your lists on the dashboard.
Click the Messaging icon in the list navigation bar in the top right hand corner of your list homescreen.
Below the green New Message button, you'll see a link that says Scheduled, click on that link. This is where all your after opt-in messages that are sent to new subscribers are stored.
To resume a specific scheduled after opt-in message, click the Resume link corresponding to the specific message you wish to resume.
When you click Resume, the message will be resumed and all new subscribers will receive that message after they subscribe.