Up until a few months ago, everything was going fine on my website. But then all of a sudden unusual symbols started appearing on the webpages. This symbols appear as �. But when i go to edit the page through cPanel file manager, they appear as �. I originally ignored the popup message when trying to edit through HTML, much to my regret, as now i am missing an index.html file. Yet when i go to edit the file, and type in any code at all, it simply disappears when i attempt to edit it once more.
To add to this, i have just noticed that these symbols appear on some pages, yet not on others.

My questions are:

Why are these symbols appearing? - They are appearing in place of symbols such as quotation marks (" or ') as well as dashes ( - ). Note that my current encoding is set to iso-8859-1, but the original popup message suggested to change it to utl-8.

How do i correct the error of the symbols?

How do i replace the missing index.html file? (of which i have a backup of the code)

Why are they occuring on some pages and not others?