I took this domain over, but don't worry!
If this is your domain, here's what happened: you setup your domain to point towards Github Pages, but probably forgot, didn't have time, or gave up and never actually setup a GitHub page.
Here's how to fix it and take back your domain: To take this page back, go to your domain registrar and delete the CNAME record for this domain. For example, if the domain for this page is "github.example.com", there should be a CNAME record for "github" that points to something like "mypage.github.io". Delete that, and now you can add in another record.