I just read a question about the Liaden books. It had no tag and I wasn't able to add one because my rep is too low. But we need one. The series deserves it!
Tags that are on a single question are automatically erased after 6 months (the presumption being that it's a typo or a nonsense tag). I suspect that's what happened here. Unfortunately, there is no advance notification and no way to declare that a tag is legitimate. The only remedy is to ask another question with the same tag.
For future reference, for an uncontroversial request like this, the quickest way is often to ask in chat; even if there's no one around, the next person to visit is likely to read back a few messages.