You must have a layer called “Floating selection”. This probably happened because you had a selection active (even a Select All) and used a transform tool (probably a rotation)(*)? The result is a floating selection. You can use Layer > Anchor (or the anchor icon at the bottom of the Layers list) to replace your original layer with the rotated version.
(*) With the the “Rotate” tool, using the Layer > Transform > Rotate ... entries doesn’t.