Sometimes after a force push or deleted ref commits can become unrefenced. If you no longer have access to a machine which contains the commit you'd normally be unable to access the sha.

However if you host your repository on GitHub or BitBucket the SHA will often still be available for a period of time until the repository expereinces a gc event. This tool sends the tag creation command directly to the git server and bypasses local client checks that prevent the creation of tags for non-existant shas.

ruby git-create-dangling-commit-tag.rb <SHA> <New Tag Name> <Git SSH Remote>
git fetch --tags
git checkout <New Tag Name>