Ask HN: How do/would you deal with your website being hacked?

I'm not super technical and it's not something I'll ever have to deal with but I'm curious; when a website is compromised how do you find out what they did and what data they took? Presumably the attacker will try and hide their tracks so I'm interested to know how you get a full understanding of what they did.

Additionally, is this something you prepare for, as part of a disaster recovery plan so to speak and what is your plan of action should an attack be carried out?