Start at this site:

https://www.facebook.com/hacked

https://www.facebook.com/help/131719720300233/

Here’s some additional info:

https://www.wikihow.com/Recover-a-Hacked-Facebook-Account

https://sheley.org/2021/08/your-facebook-account-is-hacked-this-is-how-to-fix-it/

https://hacked.com/how-to-recover-a-hacked-facebook-account/ 

https://www.lifewire.com/help-my-facebook-account-has-been-hacked-2487299

https://www.zdnet.com/article/facebook-virus-or-account-hacked-heres-how-to-fix-it/