Is the leaf rolled up like a burrito? Can you unroll it and find webbing  and an insect inside? I suspect a larvae or insect has used your hydrangea as a nursery and laid eggs on the leaves then rolled them up to protect the eggs or hatching larvae inside. The other leaf in your photo looks healthy so I will rule out lack of water or poor soil. The best cure is to pluck off every rolled up leaf and destroy the insects inside. This  problem is seasonal so in a few weeks the hydrangea could put on new growth and still have a summer of blooms. Keep growing, Marianne Binetti