Lyn, in the Page & Layer gallery if you open the page to Layers mode, do you see a Page background layer and if you expand that, does it have a Rectangle?
Is the layer locked?
If not, you might have have deleted the Rectangle or the entire layer.
Right-click on an area well away from your design and pick Pasteboard background > Reset.
This restores the defaults without touching your design.
Acorn
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