There is this one option in Elementor that sometimes creates confusion among our users and it is Page template: Elementor Canvas. Usually new users to Elementor switch this option when they try to figure out how Elementor works.
What does it do?
When you activate this page template, it turns off all of the theme features, like theme header, theme footer, and layout for content, leaving only Elementor canvas. It can be useful in some situations, but only for users that know what they are doing.
How to fix it?
Edit page, where you see this issue happening and switch Page template to default. You can do it either from Elementor or by traditional edit. Below screenshots should explain how to do it.