SHELL

git undo rebase

git reset --hard ORIG_HEAD

git undo rebase

git reflog

revert rebase

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}

git undo rebase

git reset --hard HEAD@{5}

git undo rebase

git reflog # determine pre-rebase head number, N
git reset --hard HEAD@{N}