How to restore the Public user account?

I would install a fresh copy of Kentico 11, set the same hash salt string as your original project. Then export the user account and import it into your project.

Thanks Juraj. So how could the default public user account got changed? From CMS desk the username "public" is disabled so people cannot make change to the username. Somehow in our case the username field turns blank suddenly for this public anonymous account. Someone hacked in? otherwise I can't figure out how this account username can be modified. Unless someone did it in the database?

I do not know your project - so either somebody hacked the old version or there was made some change directly in the DB or through the API.