Unblocking does not automatically re-friend them. They will need to send you a new friend request, or you must add them back manually.
If you blocked a user and want to find them to unblock them, follow these steps: how do i find blocked people on snapchat
To find people you have blocked on Snapchat, navigate to your and look for the Blocked list under Privacy Controls . This private list allows you to manage restricted contacts, review past blocks, or restore connections by unblocking users. Step-by-Step: How to See Your Blocked List Unblocking does not automatically re-friend them