once a fb ad or campaign is deleted, it is gone forever and cannot be directly restored or undeleted. however, you can reuse the exact post by finding its post ID inside your page posts manager or via the deleted ad's history, then using that ID to create a new ad without losing existing social proof
I think once you delete the ad, there is no way to bring that back. You can also check the deleted history section to see if the ad is available there otherwise you need to recreate. If you have the old image, video or ad copy saved somewhere, you can recreate it easily and I always try to duplicate ads before making any changes, just to avoid this kind of issue.
If it was permanently deleted, you may need to recreate it. Try to copy the original settings from your campaign history or reports.