Page not found in preview and most viewed

Hi there! Im a beginner with wordpress and i was trying to fix my SEO ranking and figuring how to disable posts that would appear in search engines that had already been changed. So i was trying to see if I could redirect them to the new version of my posts. As well, before that I changed the titles of my page names and categories, as well as some of my posts permalinks -because i changed the title name- Then i went into permalinks to see if I could redirect my old posts that appeared in search engines to the new ones. And changed some stuff -like the permalink structure (from /%category%/%sitename% to /%sitenam%/ )and then all of my posts and pages turned to 404. I tried adding a code that Yoast suggested to add in my ht access file ro redirect the old permalinks to the new one but that didn’t work. So i eliminated it from my code editor, and changed my permalink structure to its original structure, clicked saved and my posts came back, yet there is one page in particular that is not appearing, which today had appeared, till this moment- I’ve tried everything, and it won’t come back, and it appears on my site as most viewed. Could anyone help me or suggest anything that could make that page appear, even when i click preview it says 404.