If you have created a floating selection, a temporary layer, called a «floating layer» or «floating selection», is added to the layer stack. As long as the floating layer persists, you can only work on that layer.

To work on the rest of the image, you must «anchor» the floating layer to the former active layer with the Anchor Floating Layer or Mask command; or convert it to a new layer with the To New Layer command. The anchor menu command is only visible if a floating selection is present.