I think you can complain to the authorities like police since stealing something which is not your's is a fraud and you cab sue him/her.
Another thing you can also complain for tge support services of the electronic wallet that your btc was held.
And you think what you say is useful or not, I don't think police and law and security organizations can help you with these problems because bitcoin is probably not a legitimate asset in countries and of course, when it's stolen, the government won't help you, that's not their problem, that's your problem. And even if countries accept bitcoin as a legitimate asset, your complaint is still useless and cannot help you regain assets because the system and features from bitcoin are so special, organizations and countries still haven't found a solution to manage and control it.