The search icon currently replaces the whole “File” and “Assets” view in the sidebar. You can only get back to the UI by clicking the X icon. At the same time the search only searches through layers and content, to find assets you have to click on the “Assets” tab and use the searchfield inside this tab.
It would be more intuitive if the search wouldn’t replace the whole sidebar content. It is also more consistent to have the searchfield inside the “File” tab.
BTW: It is also counter intuitive that the search icon is replaced with the library icon, when switching to the “Assets” tab.