Maybe you made a very tiny selection somewhere on the page with the select tool? If you did, you might not be able to see it and you won't be able to draw anywhere outside of it. Just take the select tool and click once on the page without dragging to make sure anything selected will be deselected. If not, above the area where you change the size of the brush there's a series of buttons for pre-made brushes in a row. Try clicking one of those and then drawing. It seems to me that's worked in the past.

or ctrl+d