Can someone please help me understand what\'s happening here?
I start with git log --oneline, which spits out:
4df9421 (HEAD, master) mo
You should accept VonC's answer. To complete it :
The "missing commit" was not pushed to the server, or saved in another branch. Your git reset --hard thus deleted it.
Fortunately, git has some sort of magic undo stack : git reflog. Check VonC's link to figure out how to get it back.
Be careful when you use git reset, this command can destroy commits.
As a rule of thumb : before you manipulate your history, make sure you still have some way of getting back to where you were. My local repositories are crippled with backup, orig and wip branches, which I clean up every month or so.