Was the document last used with the same Zotero library on the same computer (or another one via Zotero sync)?

Generally this happens if you transfer a library using export/import, which breaks document links.

Highlight a missing citation and select Toggle Field Codes, and then paste in here the URL that shows up.

Also provide a Report ID if possible.

An item with that id doesn't exist in your library on the server, and presumably it doesn't exist in your local library (which is likely the same) if you're getting that error message. The Zotero library that document was created with doesn't appear to be the same as your current library.

Note that what matters is the id stored in the document. Even if the item looks the same to you, if the id is different (because export/import was used rather than Zotero syncing, for example) the citation will show up as missing. Zotero 3.0 will improve this situation somewhat by storing metadata within the document itself.

Unfortunately we can't tell you that because we don't know what happened.
The best, guess, as Dan says is that

Generally this happens if you transfer a library using export/import, which breaks document links.

You may be faster re-linking the citations in your document. If you install Zotero 3.0, which just came out, that process should be much faster given the improved word processor plugin.