If it's been drought stressed, yes, it's about time for dieback. Mine here has bloomed and is starting to look somewhat ratty, probably in a week or it it'll turn yellow for me. However, I'm certain of it's return next year.

Mine is still happy and healthy looking, and blooming its many heads off. Is yours in a moist, shady location?

The location is shady and moist. There are young hydrangeas close by that look healthy so I don't think that could be the problem.
I'm afraid it must have got a "bug." There is nothing left but a black skeleton.