Brush disapearing to thin line?

Having an issue with understanding the behaviour of the brush. I changed a setting somehow so that my previous brush stroke has just became a path. No matter what I do, the brush strokes won't come back. Any new brush stroke I make automatically becomes a colourless path. Stroke weight and colour don't seem to make a difference, and the colour automatically changes when I try to use the brush. Any help would be appreciated!