I ended up with a detached head today, the same problem as described in this question:
"git push says everything up-to-date even though I have local changes"
As far as I know I didn't do anything out of the ordinary, just commits and pushes from my local repo.
So how did I end up with a detached head?
Any checkout of a commit that is not at the tip of one of your branches will get you a detached HEAD.
See committing with a detached HEAD
When HEAD is detached, commits work like normal, except no named branch gets updated. (You can think of this as an anonymous branch.)
For example, if you checkout a "remote branch" without tracking it first, you can end up with a detached HEAD.
See git: switch branch without detaching head
It can easily happen if you try to undo changes you've made by re-checking-out files and not quite getting the syntax right.
You can look at the output of git log - you could paste the tail of the log here since the last successful commit, and we could all see what you did. Or you could paste-bin it and ask nicely in #git on freenode IRC.