No Idea, hopefully it will not happen to mine.
Your best bet is to call Home Office Number for the Nationality Division in Liverpool. They usually require copies of all pages on the Passport certified by a solicitor. If you send your passport it usually take 6-8 weeks to return but your case might be different as it was their mistake and they may do it quickly. I wouldn't count on it.
here is their Number for Nationality Division. They might guide you properly if this number cannot help you.
0845 010 5200