Mine are slow to flower this year and don't really look good at all. My H. paniculata Vanille Fraise doesn't even show signs of wanting to flower. I'm also house and garden sitting for someone who has also had hydrangea problems - he's cut his back as it looked very withered despite good watering etc.
I'm inclined to think that it's the weather. I think it's come as a shock to some of my garden (although the roses look awesome)!
Try saving yours as you've had it such a long time and it would be a shame to see it die. Does it have any shade from the sun?

Have you tried giving it a good soaking? Have you scratched the bark on the affected branches to see if its still green underneath?

Looking at the pic the top of the hydrangea looks to have suffered scorch due to the hot sun where as the bottom stems are more shaded, I agree with steragram all the hydrangeas I deal with for clients have been heavily watered and due to the severe heat and sun I have shaded them which has kept them in good health, don’t be down cast you may have to cut some of the stems back in the autumn and I am sure it will be better next year but may not flower very good until year after, I have adopted this strategy with other plants which have struggled in this heat.

Believe it or not, Hydrangeas will grow here in the desert, but they have to have shade most of the day, because of the heat. I'm afraid that you folks have just gotten a taste of the desert--I think that you probably would have preferred prickly pear preserves, or some mescal! ;)

Can I side track from the question pleas Sukipop?

Interesting Tug - didn't know mescal came from Agaves. I thought it just came from Lophophora williamsii (which i had as a teenager but it was too expensive to try eating it...)

Nope, Stera, that's mescaline, a hallucinogenic with the usual horrific side effects and addiction issues. Its relationship with mescal is as follows:
Mescal was originally the wildcat "white lightnin'" form of tequila, made from wild agave species, rather than the long cultivated Agave tequilana and A. weberi. Many wild agaves contain trace quantities of mescaline, and some of the rougher "brands" of mescal have more "kick" than you might want! In the more respectable brands on the market, the mescaline has been processed out, sometimes by putting a "worm"--a beetle grub that commonly lives on agaves--in the bottle to absorb the excess mescaline. That's why party animals that "eat the worm" have been known to end the party in flashing lights.

Too much hot sun. Hydrangeas do best with some shade.