Golly!! That looks very much like a plant that has received NO water recently but is that possible? If not a water issue, I'd be suspicious that the "persons unknown" that removed the flower heads also poured something disagreeable into the planter as well.

This problem can also be caused by a plugged drain allowing the roots to be drowned and no longer able to conduct the water. Al

The above is very true but the timing of the occurrence seems extremely coincidental. The hydrangea is a good sized plant and seems to be well-established in that container and I would have thought that lack of proper drainage would have shown up earlier (doesn't take long to plug up a drain hole). Concrete aggregate planters typically have pretty generously sized drain holes as well.

Wow. These two guys are not happy. ;o)

Yes, it looks like a watering problem. I have not used that material but it may be made worse by rocks. Rocks absorb heat and release it slowly in the summer, which stresses/burns the roots in contact with the sides of the pot. That is why you do not want to use rocks as mulch either. Once summer temps moderate, any root damage remains so the plant's look does not immediately improve. Some people prevent this issue by putting a cooler wooden pot around the pot that gets hot but, this rock pot looks nice and I would hate to suggest to cover it next summer by covering it with a wooden one. Heat problems can start with temps around 80-85F (26-29C) and winds alone can also make it worse.

Moving the shrub to another pot of different material may be best and remember to select a location that is not windy (wind dries out the plant just as bad as high temperatures) and that provides morning sun and afternoon shade.

The leaves will be dropping soon so I would not accelerate that. Just try to keep the potting soil as evenly moist as you can. Check for windy conditions and check the soil moisture twice a day to make sure it does not need more than one watering per day.

Keep an eye on the soil pH too since concrete and frequent watering can make the potting soil alkaline.

I am not sure why someone would bother stealing all the blooms. :o( At least over here, they are easily and cheaply purchased at grocery stores! Is this area "open to the public"? There appears to be a small sitting wall on the first picture,

You should also be getting ready to move it elsewhere for winter? Or is it mild enough and you leave it potted outside? Just wondering because today I started adding a tbsp of vinegar to a gallon of water to kill gnats on potted plants soil (I will be bringing them inside for winter in 2-3 weeks depending on the temperatures).

I greatly appreciate all the prompt and insightful responses! The 2 hydrangea plants were absolutely healthy-looking and gorgeously full of blossoms for the past 2 years. The climate is moderate enough in this area, that plants do not require protection from winter temps. In Feb/March we have pansies sprouting in window boxes :-)

Both plants were thoroughly watered by our neighbor days before the blossoms were removed and the plants wilted in unison.

Drainage is not a problem, as water is visible as it flows out the bottom of the planters. I will take a soil sample to a lab to verify if "something disagreeable" was poured into the platers, as well.

Many thanks, again, for all your insights.