my website went blank after added a feature called ''Prevent Access to Default WP Files&#0

Hey. I recently started to use a plugin called ''all in one WP security and firewall''. And I started to use a feature called ''Prevent Access to Default WP Files''. And my website went blank. I have tried to remove the plugin with ftp, but it didn't work, still blank.
Hopefully you can help me.. Tho the feature worked at my other subsites, but I don't know why my mainsite can't handle it.

greetings , Marcus