Do they appear in the text of the document and are only missing in the bibliography? If so, most likely at some point citations got unlinked.

It's hard to say what happened, but generally this wouldn't be Zotero's fault. Zotero itself does not remove citations from documents at all. The worst that Zotero could do is unlink citations, but only if the user pressed the Unlink Citations button and confirmed the action, but that would still leave plain text citations inside the text.

If you are using a footnote/endnote citation style, it is possible you have accidentally removed the footnotes with citations by accidentally overwriting some portion of them.

In any case, unless you have an earlier backup of the document, you will have to reinsert those citations manually.

If you open Document Preferences from the Zotero tab in Word are you storing citations as Bookmarks? If you are, that may have contributed to the issue. Bookmarks are prone to become corrupted and you should use Fields unless you absolutely need to collaborate with someone with LibreOffice.

In that case there is little we can say on what happened. We have never had any user report anything like this, so it's extremely unlikely to be a Zotero bug, although technically feasible. It's highly unlikely to happen again, but if it does and you find out what causes it, please tell us and we will fix it.

Otherwise, I'm sorry to say this, but you will have to manually reinsert your citations, unless you have an older backup of the document that you can restore them from.