At the top of your campaign dashboard, there is a funnel icon. After pressing this icon, choose 'Archived Campaigns' from the drop-down. This will list all of the archived campaigns within your project.
Press the 'Actions' button on the campaign you wish to unarchive, and then 'Restore Campaign'.
This will restore your campaign back to your campaign dashboard.
Note: Any sites within the workflow of an unarchived campaign will still count towards your opportunity deduplication.