There is both a film rewind button (on the bottom of the camera) and a film rewind crank (on the top of the camera).
When you are at the end of the film, you push the button, and then rewind with the crank.
As mentioned, to confirm that the film is winding properly, take up slack with the rewind crank and watch to make sure that the crank turns when you wind the film.
I used to sell the C35 - in 1980 or so.
By the way, 24 exposure rolls are still around, although I'm not sure how widely available they are.
And the C35 manual only refers to 20 and 36 exposure rolls!