Since, you do not see anything with ls -l .git/refs/heads , it implies that you no longer have master branch (may be you deleted your master branch unknowingly as you said above you weren't sure when you accidentally created your branch.
To go back to your original master branch, check your git log for when you created the new branch. once you obtain the commit sha , recreate you master branch with "git checkout -b master ". This will take you to your default prompt.