If I ignored someone on Snapchat when they added me, how do I unignore them?
To unblock someone on Snapchat, go to Settings menu and scroll down until you see Blocked. Tap the username/ID you just blocked and then tap again on the X corresponding to the username. Inform the user again to add you or add him again and that's it.
For more information, access the following website: https://support.snapchat.com/a/block-friends