Big problems after changing admin

I wanted to help somebody after they were hit by brute force attacks from China.

First things first, they still had the default admin login.

So I changed that using the WordPress functionality for that within users.
I didn’t do it via phpmyadmin because they didn’t know their password anymore.

It seemed it was ok. But it wasn’t. Images are gone, links to images are broken, posts and pages ended up in trash instead of live. And I don’t have an user anymore with id=1 in the database.

All of a sudden some people couldn’t login also anymore.

So lots of problems. How do I solve this mess? Any suggestions as, you’ve guessed it by now, their shitty webhost didn’t have a backup also.

Thank you in advance