curve extrusion doesn't work anymore Not sure what I've done, but I am unable to undo it. When I extrude the curve, it summons a white dot instead of adding a joint curve.

I am not sure where I did wrong here.
You must have accidentally pressed H (hide), it has hidden the last extrusion, and when you try to do a new extrusion, as one already exists, it just creates a single vertex. So try to press altH.