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@{5}
in the ref log:
git reset --hard HEAD@{5}
Also rebase saves your starting point to ORIG_HEAD
so this is usually as simple as:
git reset --hard ORIG_HEAD