Undo a git rebase.

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

$ git reflog

Then reset the current branch to the old commit. Suppose the commit was HEAD@{10} in the ref log.

$ git reset --hard HEAD@{10}

In windows, you may need to quote the reference:

$ git reset --hard "HEAD@{10}"