Git commit against tag with no branch

Because your commit isn't on any branch, you can't see it in the working directory unless you checkout that specific commit, using its SHA1. You can find the commit by looking at the reflog which tracks changes in what you have checked out from the repo. If your tag was XXX you'll see something like:

$ git reflog
7a30fd7... HEAD@{0}: checkout: moving from master to XXX
ddf751d... HEAD@{1}: checkout: moving from 96c3b0300ccf16b64efc260c21c85ba9030f2e3a to master
96c3b03... HEAD@{2}: commit:  example commit on tag XXX, not on any branch
7a30fd7... HEAD@{3}: checkout: moving from master to XXX

That tells you the SHA1 that you would have to checkout in order to see your commit in the working directory.

$ git checkout 96c3b03
Note: moving to "96c3b03" which isn't a local branch
If you want to create a new branch from this checkout, you may do so
(now or later) by using -b with the checkout command again. Example:
  git checkout -b <new_branch_name>
HEAD is now at 96c3b03... example commit on tag XXX, not on any branch
$ git checkout -b newbranch
$ git branch                #lists all branches
    feature1
    master
  * newbranch

This all seemed a little weird to me at first, until I realized that git checkout places all the project files as of a particular commit into my file system (working directory). In effect, the working directory acts as a browser on the local Git repository. So your changes haven't been overwritten in the repository, they're just not being shown in your working directory when you've checked out the master.

Yes, they'll be in reflogs.

You can name the branch at any time like this:

git checkout -b my-branch-name

Alternatively, you can merge the commit back into master without a new branch by finding its SHA1 (using git reflog as above) and then:

git checkout master
git merge SHA1