I would check the soil for too much moisture. Unfortunately, in extreme temps like we are experiencing right now there is a tendency to over water plants. In this heat plants may transpire faster than they can take up the water during the heat of the day, even if there is available soil moisture. In this case, plants may wilt some during the day, but then perk back up in the evening or over night. If the plant is wilting some and you water, and the plant perks back up within a couple hours then it was a lack of water. If it is wilting and you water it and it does not perk back up in a short amount of time then it could be too wet. 

Hydrangeas won't tolerate sogginess so I would dig a hole near your plant, about 6 to 10 inches deep to see what the soil moisture is like deeper in the soil. If soggy, stop watering for a while to allow the soil to dry out a little bit. To help reduce some of the heat stress while it is trying to adjust or recover from potential root injury, you might consider shading it with something like burlap draped over some stakes or use some other material just to shade it for a while.