Go into Theme, Customize, and there should be a “Menu” choice. Select an existing menu (if still there) or create a new one.
When you change themes, you often have to re-specify the menu again, even if it already exists.
The ‘three-bar graphic’ (AKA “hamburger”) is shown at certain (usually smaller) screen sizes. That is a function of how the theme does the CSS for smaller screens. Most will have CSS code that will turn a horizontal text menu into a Hamburger icon that you have to tap to see the menu choices.
It could be that you changes some CSS (in Theme, Customization, Additional CSS) for the CSS class used by the menu that caused it not to display the Hamburger icon. Or you could have deleted/de-specified the menu in the Customization, Menu area.
YOu didn’t specify whether you had changed the theme code. Not a good idea, BTW. If you want to re-code parts of the theme, then Child Themes are the way to go. You’ll find lots of info in the Codex/Developer area (https://developer.wordpress.org ), along with lots of googles/bings/ducks on Child Themes.