Your Blogger blog uses two different yet related HTML / XML templates, in formatting and displaying your posts.
The blog template defines objects that are consistent for the entire blog - and the post template defines objects that are consistent for each post (though the content of each post will vary, from post to post). You can change the blog tempate, when you wish.
The post template, when the blog is published, becomes part of the blog template - but the two are maintained separately. This lets you change your blog template (and the page layout) without affecting the post template (and the posts layout), and vice versa. When Blogger makes changes to post related features, they change post template code.
The blog template is configured using the dashboard Layout "Blog Posts" wizard - or the Template - "Edit HTML" Template Editor.
You can add / arrange objects on the page, and re locate any object that you want, anywhere on the page (with a few exceptions), using a GUI drag and drop process. Wherever you drag it, the object becomes part of the page, and displays in that location on the page. And when the page is displayed, the object in question is displayed there, with one copy of each object / page.
When there are problems with any of the blog content except the posts themselves, and the problem can't be resolved by editing a widget or using the Layout "Edit HTML" wizard, the blog template is possibly corrupt.
When you have a corrupt blog template, if you didn't back the template up previously, you'll have to use the "Pick New Template" wizard, and get a fresh copy of your template. Or copy it and paste it into the "Edit HTML" window. Note that each widget includes a quickedit function (the "screwdriver / wrench" icon), which is also part of the blog template.
The posts template is configured using the "Configure Blog Posts" widget in the dashboard Layout wizard. You can arrange (but can't add) objects using the GUI blog posts widget. If you want to add an object to be part of the posts, and display with each post, you either add it into each post manually, or using the (Settings - "Posts and comments") Post Template "Edit" link.
The Post Template Editor (please, do not confuse this with the "Template Editor") uses HTML code, not GUI - and it works for new posts only.
When there are problems with the posts, and the problems can't be resolved by editing the posts using the post editor, the post template is possibly corrupt. This can sometimes result from unwisely applied tweaks, made by the blog owner.
When you have a corrupt post template, you have to reset the post template. This requires that you delete / re add the post template code from / to the blog template, using one of several possible techniques. Note that the post template also includes the post quick edit function (the "pencil" icon), the comments, and the labels - all elements which are different for each post.
Be sure to backup the template, before and after resetting the post template.