Please check this tag untagged. It looks like one of our tag got deleted by mistake. Can we know which tag was deleted and can we restore/recreate the same tag?
I talked to one of the Community Managers, and they said there's no way to find out what tag got deleted to make a question untagged, short of using web.archive.org to find an old version of the question. Or I suppose we could ask the people who posted the question in the first place.
Note that if a tag is only used in a single question, it will be deleted after 6 months. You can protect a tag like that from deletion by creating a tag wiki for it. Here is a script that will tell you what tags are only used on one question.