Usually this happens after an update, but the cause isn’t clear. Fortunately, the panel can be moved back without too much trouble.
There’s two ways to move the panel back under the post content. The first and easiest is to use the navigation arrows to move the panel back where it was.
Using the Classic Editor.
The other way to move the panel is to install the WordPress Classic Editor plugin, and use the mouse to drag the panel back under the post content.
When the panel is back where it should be you can deactivate the Classic Editor plugin and the panel will stay where you’ve put it.
