One easy way to get wilting and drooping is to transplant a hydrangea. The transplant rarely moves all of the roots so the plant becomes as sensitive to heat stress as it used to be when originally planted. Drooping happens due to heat stress: temperatures at or above 85F, not enough water, summer sun hits the plant in the afternoons or evenings, windy conditions, low humidity, etc.
With less of a root system in the new spot, it will loose moisture through the leaves faster than the roots can absorb more water so these episodes will occur often as temps hit 85F or more in the Summer. The most you can do is make sure that the soil is mulched and kept evenly moist, that the plant gets shade in the afternoons/evenings, that the plant is not in a windy area. After it becomes established, it will not have as much trouble in the summer but it will always complain during the worst of the summer.
In the northeast, the summer sun tends to be weaker so it may be possible to put the plant in full sun provided the soil drains well, provided it has 2-4" of mulch year around, provided it gets sufficient water... and sufficient shade if it complains (use an outside chair -for example- to provide additional shade). My late mother in law had a Nikko Blue on a western exposure near the border of Mass and N. H....
It is rare to hear someone moving them to full sun. How much sun was it getting before and why did you move it to that spot in full sun? If the plant used to do well before in full sun during the summer then acclimate it to this new location still in full sun by giving it TLC (extra shade, extra water, etc) as needed but if it was getting morning sun only or dappled sun, be aware that the transition will take several years. My late mother in law had a huge Nikko Blue on a western exposure near the border of Mass and New Hampshire. She would tweak the amount if water when temps hit 85F and do extra waterings if she saw wilting/drooping in the morning hours (6-8am).
With my clayish soil, I normally supply hydrangeas with 1 gallon of water per plant in Spring, increase that to 1.5 gallons in May (that is when we start hitting 90s down here), then 2 gallons when temperatures are regularly above 95F thru September. You can use those amounts as guides and tweak as necessary. Just make sure that a while after watering, the soil should feel moist to a depth of around 8" in all directions. So, insert a finger (in several places) into the soil to a depth of 8" and see if it feels moist. Tweak as needed for your conditions which may require even much, much more water than those numbers.
I would also prune off the blooms this time only in the hopes that this helps use the water in the leaves instead.
This series of hydrangeas blooms on old and new wood. Are these blooms from the old or new wood. If they are from the old wood, you may get more bloomage later in the Summer as the new wood becomes taller and older. But if it does not bloom (due to transplant and messing with the roots), I would not worry this time only.