I just had the same thing - my document got unlinked. I did nothing of what explained in that page for "known causes". I did not open the file in two weeks, and after opening it I made a copy of it. Thats all.
OK, please always start new threads for new issues — the thread you posted to was about Google Docs, so the troubleshooting page had nothing to do with your issue.
See Existing Citations Not Detected.