How to get back your identity?

Hi all,

Someone has stolen my MC's identity to the point where everyone believes that the antagonist is my MC. Unfortunately, I've got no idea on how my MC will get it back.

FYI, the antagonist stole my MC's identity in order to take over his business and get rid of him.

Thanks.

What form did the identity theft take? Mundane stealing of credentials? Extreme plastic surgery? Grand Theft Body?

Why can't he just run in and say "I'm not this person I look like, I'm really me! I can prove it with an obscure question that only I would know the answer to"?