It appears you rejected a real friend, and now you are sad. It's easy to fix - just follow the steps below!
To accept a rejected friendship - you need to go to http://untappd.com/user/USERNAME (where USERNAME is the person you rejected) and then click the Confirm Friendship method. Not sure what their username is? Read this.
This can be done on the web/tablet:
Or the mobile web (where this no Confirm Icon - just the Yellow +)
If you see grey on the web or mobile (as shown below), it means that the other person has rejected your request, not you. If you are both seeing the grey button, read this.