Hidden pages now visible to all

My WordPress Site was set up to be accessible only to those who are given user logins and passwords (i.e, unavailable to the public). Suddenly two pages are visible to anyone, not just users, although others are not.

All the pages were set to visibility: public. I tried to fix one page by setting visibility to private; that didn’t work, but it displays a “Whoops! Page Not Found”message. I tried the same with the other page (set visibility to private) and that had no affect; it remains visible to all.

How to I reset all the pages for the entire site to be open only to authenticated users?
J-M Thomas
https://www.pvopoa.org