My Site was hacked

The home page of my site https://timeoutbookings.co.za/ was hacked by someone saying AHG Was Here !!!.Greetz

I managed to change the page back by using the autosave and restored a previous page.

My question are;
1. How do I determine what damage was done to the site.
2. How do I know if other pages were hacked (without physically looking at all pages)
3. How did they get access to my admin panel
4. I am not sure how to remove the hack from the auto save page
5. What steps must I take to prevent this happening again.

Below is a copy of the html code from the hack…..

[Hacked code moderated]