Anti theft mode

So I go to get in my car an start it up an it won't start. Not turning over what so ever. Mechanic has a look checks battery,starter motor etc an can not find anything mechanically wrong with it. Anti theft light is coming up on dash. Then a message on dash saying I need to replace battery in fob.. change battery but makes no difference. I see a few posts online saying to disconnect battery,so I do that but makes no difference either. What's going on??car is a 2011 Holden Cruze