If you have difficulty reading the chalkboard from the back row, what vision problem could you have?

Refractive error: If you could do this at one time. And the eyes have not been injured or gotten any illness, then the most likely thing is that you have acquired a need for eyeglass correction, most likely myopia (nearsightedness) and perhaps a little astigmatism. See your ophthalmologist to get it measured and have the proper eyeglasses prescribed.