During a page EDIT: For reasons unknown to me, sometimes a spacer or reusable block will show up as an “Attempt Block Recovery” element: This happened for a group of Spacer Blocks:
Chances are your page looks fine, and if it does, you could probably just leave it until you have time to investigate.
I decided to fix a series of spacers on a page. Clicking the blue box: “Attempt Block Recovery” does restore the spacer, BUT it restores it at the default height of 100 pixels;
These spacers had to be manually reset, one-by-one, to the original height of 20 pixels:
