I had this happen to me a while back on a different version.
By design, if the viewport is too small to display the sidebar, it slides down under the content, as it does on a smart phone.
So if you've lost it on a desktop or lap top, try checking below first. If it's now above the footer, go to Appearance >> Page Setup >> Responsive and check your break point settings there.
By design, if the viewport is too small to display the sidebar, it slides down under the content, as it does on a smart phone.
So if you've lost it on a desktop or lap top, try checking below first. If it's now above the footer, go to Appearance >> Page Setup >> Responsive and check your break point settings there.