Please follow the below instructions to "Resubscribe"
 
1. Please click on contacts. 
2. Please apply the filter "Email is and enter the email address of the contact"
3. Please select the contact once it is listed
4. Please click on bulk action and select Resubscribe and click OK.  It will send an email with the button "Confirm   Subscription" The user needs to click on that button and he will be resubscribed.