Can’t get to the WP dashboard

My WP is in a subfolder (/nieuw) and I’ve edited the index.php
It used to work all fine. Recently i suspect there was some hacking activity. I can now only log in with my admin credentials via the Woocommerce my-login page.
I do see the dashboard menu and they are pointing to the folder – such as https://joopsmits.com/nieuw/wp-admin/post-new.php but all clicks say:

NO RESULTS FOUND
The page you requested could not be found. Try refining your search, or use the navigation above to locate the post.

The URL is show in the address bar though.

Any way to get in the backend again?
Rob

The page I need help with: [log in to see the link]