cannot push to github: everything up-to-date

git commit 

Will show you what files are on your local machine, uncomment what you want to upload and

git push origin master

Because git add * did not work for me (even if it didn't returned errors).

After you change files, you need to

git add

them prior to

git commit

.

Not sure why people are down voting the guy with the correct answer. For me adding my email and name did resolve the issue. Although the commands are not correct.

git config --global user.email "[email protected]"
git config --global user.name "Your Name"

git branch -v indicates that my commit was on (no branch). As for the add, I initially commited the changes through Eclipse (with the git plugin)...when I do git add from the command line, it doesn't seem to do anything

That means you are in a DETACHED HEAD mode.
You can add and commit, but from the upstream repo point of view (ie from the GitHub repo), no new commits are ready to be pushed.
You have various ways to include your local (detached HEAD) commit back into a branch, which you will be able to push then.
See:

• "Not currently on any branch + git commit + checkout when not in any branch. Did I loose my changes?"
• "detached HEAD explained".
• "Git: How can I reconcile detached HEAD with master/origin?"
• "Git Lesson: Be mindful of a detached head"
• "Git Tip of the Week: Detached Heads"

The OP mentions this article in order to fix the situation:
"git: what to do if you commit to no branch"

all we need to do is checkout the branch we should have been on and merge in that commit SHA:

Note that instead of merging the SHA1 that you would have somehow copied, you can memorize it with a script, using head=$(git rev-parse HEAD):
See "git: reliably switching to a detached HEAD and then restore HEAD later, all from a script".
Then you can merge that detached HEAD back to the right branch.