The easiest way would be to find the head commit of the branch as it was immediately before the rebase started in the reflog...

git reflog

and to reset the current branch to it (with the usual caveats about being absolutely sure before reseting with the --hard option).

Suppose the old commit was HEAD@{2} in the ref log:

git reset --hard HEAD@{2}

In Windows, you may need to quote the reference:

git reset --hard "HEAD@{2}"

You can check the history of the candidate old head by just doing a git log HEAD@{2} (Windows: git log "HEAD@{2}").

If you've not disabled per branch reflogs you should be able to simply do git reflog branchname@{1} as a rebase detaches the branch head before reattaching to the final head. I would double check this, though as I haven't verified this recently.

Per default, all reflogs are activated for non-bare repositories:

[core]
    logAllRefUpdates = true

How do you create a new project/repository?

A git repository is simply a directory containing a special .git directory.

This is different from "centralised" version-control systems (like subversion), where a "repository" is hosted on a remote server, which you checkout into a "working copy" directory. With git, your working copy is the repository.

Simply run git init in the directory which contains the files you wish to track.

For example,

cd ~/code/project001/
git init

This creates a .git (hidden) folder in the current directory.

To make a new project, run git init with an additional argument (the name of the directory to be created):

git init project002

(This is equivalent to: mkdir project002 && cd project002 && git init)

To check if the current current path is within a git repository, simply run git status - if it's not a repository, it will report "fatal: Not a git repository"

You could also list the .git directory, and check it contains files/directories similar to the following:

$ ls .git
HEAD         config       hooks/       objects/
branches/    description  info/        refs/

If for whatever reason you wish to "de-git" a repository (you wish to stop using git to track that project). Simply remove the .git directory at the base level of the repository.

cd ~/code/project001/
rm -rf .git/

Caution: This will destroy all revision history, all your tags, everything git has done. It will not touch the "current" files (the files you can currently see), but previous changes, deleted files and so on will be unrecoverable!