I was doing some work in my repository and noticed a file has local changes. I didn't want them anymore so I deleted the file, thinking I can just checkout a fresh copy. I wanted to do the git equivalent of
svn up .
git checkout HEAD^ src/
Detached head means you are no longer on a branch, you have checked out a single commit in the history (in this case the commit previous to HEAD, i.e. HEAD^).
You only need to checkout the branch you were on, e.g.
git checkout master
Next time you have changed a file and want to restore it to the state it is in the index, don't delete the file first, just do
git checkout -- path/to/foo
This will restore the file foo to the state it is in the index.