The option to set featured image has disappeared
Please, turn off all of the plugins, refresh your cache, and check if the problem persists.
Moreover, if you use a child theme, switch to parent, and recheck it.
Also, open the preferences (while editing the page), and see if you do not have a Featured image disabled there.
If I switch to parent, featured image option shows.
But then the Child Theme has disappeared so I can´t activate it again with all it´s customization (I have done a restore to recover the website).
Also I get this message: Error: Template is missing. Standalone themes need to have a templates/index.html or index.php template file. Child themes need to have a Template header in the style.css stylesheet.
Sorry, but we do not provide help with Child themes.
In our documentation, you can read:
Child theme support policy
Using a child theme usually means that you want to modify the core theme files. Any mistake in the code may render the page unusable. Please be careful when performing such actions because our support policy does not cover such changes and we will be not able to help you.
You should contact your web developer instead.