I'd been happily using inkscape for years now, but I've just a day or so ago encountered a big problem. Suddenly, any shape I try to draw (circle tool, rectangle tool, etc) will show up as transparent for both fill and stroke. It will have and retain color properties, but it doesn't show them. The object is there, you can select it, change properties, export selection, but it all results in 100% transparent images.
Lines, however, work fine. I can draw them, connect them, change stroke properties, connect them up and fill them.

This was suddenly happening on a previous version which I'd been using without issue for over a year. So I upgraded to the latest stable, and it still is happening, even with system restart. Is there some mysterious "make inkscape useless" button I accidentally ticked in the settings?

Is Opacity of the style set to 0? You have the Opacity slider in Fill and Stroke dialogue that applies to the whole object and also A slider to change Fill and stroke opacity independently.