Stuck in Weird State

Any ideas how I got into this mode? Just a efw minutes ago 2 odd symptoms have suddenly come about: when I create a sphere/cylnder/cone in object mode, their panel properties (Vertices, Rings, Radius, Cap ends, etc.) is greyed out and can’t be changed from the tool bar or F6. Also, Undo stopped working; Ctrl-Z and Ctrl-Alt-Z don’t do anything.

I haven’t changed any major settings recently, other than hitting File->External Data->Pack into Blend File, and perhaps my hand brushed over a key or two. It remains like this even when I save and reload the scene in Blender.

Both of these only happen in object mode. In edit mode, I can adjust circle vertex count and undo fine, so they’re only disabled in Object mode. Anyone?