Now can be a very stressful time to plant water sensitive plants like hydrangeas. Paniculatas like PW are probably the most sun tolerant hydrangeas but, plant any hydrangea in the middle of the summer and one needs to reeeeeally check it often. Between the hot sun, dry winds, lack of rain and the high temperatures, there is a lot out there now that can cause problems.
You did not say where are you but, in the southern states with really hot summer like Texas, consider placing it in a location that gets afternoon shade instead. Some people try to temporarily help the plants by covering them with umbrellas or chairs during the summer in their first summer in the garden.
To start, I would extract the plant and dump it in a bucket of water. Leave it there for several hours and then replant it. This will re-hydrate the root ball and not cause much of an issue to the roots. Then it becomes a waiting game to see if the plant produces new growth now or waits until next year. I had one that decided to wait until Spring leaf out time so, maybe, yours might survive. Now you cannot easily tell if it is dead but if you do not get leaf out by mid-to-late May in Spring 2017, the plant did not make it. You could try scratching some of the stems to see if you "see green" but, what you need to find out is if the roots are ok and that can be a difficult.
Make sure that, going forward, the plant is mulched at all times with 2-4" of organic mulch up to the drip line. Do not use rocks around the base of the shrub. And make sure that the soil is as evenly moist as possible... meaning no period of dry soil then wet soil, then back to dry soil, then wet soil, etc.
If you notice that the new leaves are wilting, that is fine as long as the leaves recover on their own by next morning. If you notice that new leaves are browning from the edges inwards, the plant needs water.
To determine if the soil needs water, during the early morning hours, use a moisture sensor or insert a finger into the soil to a depth of 4" and water if the soil feels dry or almost dry. Each time you water, use about 1.5 gallons of water (1g in Spring, Fall or Winter). Water the soil -not the leaves- from the root ball outwards. When the Fall temps arrive, give them 1 gallon of water. Once the plant goes dormant, give it 1 gallon of water every 1-2 weeks (but stop if the soil freezes where you live). Then resume watering in Spring 1 gallon after you notice leaf out.
PW is an excellent choice. Hope it makes it!
