Our Google Business account has been hijacked by an unknown email and the account itself has now had the main details changed. What can I do?
If you still have access to the Google (My) Business Profile, you need to check the “Users” screen (option below “Website” on the left-hand side menu if you are using the GBP manager) for the “hijacker” information.

Once you get this information, report this violation under Google My Business third-party policy here: https://support.google.com/business/contact/gmb_3p_complaints
If you don’t have access to the Google (My) Business Profile, you need to reclaim ownership of the profile, check this guide: How To Claim A Business On Google That Is Already Claimed.
I hope this article helped you.
If you need further help, I am here for you. It does not matter if it’s your business or you are a professional that wants help with a client case: