Did this hydrangea receive driving rain, more so than the other?  Are the flowers extra-large so that the weight causes the drooping?  Does a downspout send out water near this plant?  Would herbicides have been used near this hydrangea and possibly hit some of the leaves?  Do you see evidence of an animal browsing or eating roots?  These are all possible causes.  If none of these have occurred, check for insects or broken stems and let us know what you find.

Did this hydrangea receive driving rain, more so than the other? No, the two plants are in the exact same place, on the North side of the house 

We can see the photos.  This issue is perplexing, because the common causes are not occurring with your hydrangea.  Check the soil to see if it's lower  than surrounding soil so that water pools there.  Overwatering causes hydrangeas to droop, and we've received significant rain lately.  

Even small differences in the environment, such as slight variations in wind exposure or proximity to other plants, can affect how each hydrangea responds to stressors such as heat and water availability. Check for signs of overwatering or underwatering, examining the soil and root conditions, and looking for any signs of pest activity or physical damage.

Thank you 

Good luck with your hydrangea.  Hopefully, you can determine the cause of the problem.  I'm sorry we didn't have a definitive answer for you.

 I am hoping that the plant will survive the winter and come back next spring.