The old commit is still in the reflog.
git reflog
This will show a list of commits, and the “lost” commit should be in there. You can make it into a new branch. For example, if the SHA-1 is ba5a739, then you can make a new branch named “new-branch” at the old commit with:
git branch new-branch ba5a739
Note that “lost” commits will get deleted when the database is pruned.