aperture – New but old Canon 20 – 35mm f3.5 – 4.5 will no longer stop down to f 3.5

Need some info/help please. I have a 6dmk2. I bought a brand New/Old Canon 20 – 35mm f3.5 – 4.5 Love this lens, inexpensive, light and exactly what for my needs. PROBLEM: it will no longer stop down to f 3.5 no matter what I do. Anyone have any suggestions. I know I probably don’t really need to go down there but curiosity won’t leave me alone because it worked when I bought it only 2 years ago.