My Website is HACKED – How to get rid of it? HELP!!

Hi to all, maybe someone could help me how to get rid of my hacked wp blog. https://keith-web-assistants.com/blog. Any help would be much appreciated. Can anyone help me? I don’t have an idea about hacking and how that object came into my blog.

If possible, can anyone explain how does hacker access my files and why I can’t find the codes or the file that HE/SHE(hacker) used within my File Manager?

And what would be the possible solution to make that thing to never happen again(protection?).

Thank you so much