More than likely just some transplant shock and possibly fertilizer burn -- these vines are not really prone to any significant disease or insect problems. FWIW, it's not a good idea to fertilize brand new plantings. They seldom need it and it often creates unnecessary stress in what is already a stressful event (planting/transplanting). If you have semi-decent soil, there is very little need to supply additional fertilizers to any permanent plantings. In fact, I doubt I have ever fertilized my climbing hydrangea, other than what mulch may get applied at the root zone. Just keep the plant sufficiently watered (but not over-watered) and leave it be (no more fertilizer!!). It'll recover just fine.

Can you post a few pictures? Include one of this string you see. Spider mites can cause leaves to curl and grow odd-looking but will not usually kill a hydrangea. Not sure if it dried out though so I would maintain the soil evenly moist thru the end of the month or a little longer just to see if it one of those 1 year olds that wakes up unusually late. That happens a lot with many hydrangeas (leafing out late) but being so so late, this bears watching and monitoring closely. Make sure you have 3-4" of mulch thru the drip line to help maintain the soil moist longer and not let it dry easily. Do not fertilize the shrub. A gallon of water per watering should suffice.

One of my Limelight Hydrangea trees is planted in a spot which has a propensity to take on and keep too much water (Planted on August 30, 2014). It was full and ready to bloom when planted but it was starting to get a bit dry and droopy while in the container the day it was delivered to me, so I watered it after planting it that same day and it firmed up beautifully by the next morning. However, a day after that there was a very hard rain which caused the tree's leaves to droop, then wilt and yellow and die. I tried puncturing the soil to allow oxygen and it started to bounce back but another day later all of the leaves and beginner buds drooped badly. I then carefully removed the tree (root ball fully in tack and in doing so I confirmed what I thought, which was it had been sitting in basically muddy water (rotting the lowest roots to some degree I presume). I laid the tree on its side over a small barrel to protect the limbs, then I set-up and aimed an electric fan at the root ball in the hopes of drying off at least some of the moisture. Several hours later, I filled the hole with much more dirt (a mixture of existing top soil, composted soil and fertilized tree planting soil). Then I put the tree back into the ground (planted High and Dry). It rained again the next day but not too heavy or long. The tree continues to lose leaves (yellowing then dropping, etc.). Only about 50% of the original leaves and young beginner buds are still on the tree but they all remain droopy. Note: I planted two other identical trees across the yard using the same soil mixture and both are doing fabulously well (they were originally planted just a bit higher than the sickly tree). My Question is: is there anything I can do at this point to help repair the root rot that has already taken place, thus saving the growth that is limp but remains on the tree? (again I must keep that tree where it is for I have no other place to plant it and it fit aesthetically in that spot). If not, can I prune it and when can I do so in the hopes of it coming back next season.

@luis_pr you wrote: "Other species of hydrangea grow shallow, tiny, fibrous roots mostly around the top 4" only so I doubt they will push up the pavers much unless the pavers were already loose. They also need frequent watering and am not sure they will get enough water.Probably enough at first but not long term... as they start to grow under the pavers, they will not get enough moisture. The roots will then be dried out by the pavers during the day. Then at night, the pavers will release heat, just as the hydrangea roots expect some cooling. Not necessarily good conditions for the mopheads." Your prediction of what happens when they start growing under pavers and not get enough water - is interesting and sounds logical. I wonder if this is why I have trouble with some jasmins I have planted along these same pavers. Their opening is also 18"x8". I lost one last year after it lived 3 yrs and now I'm losing another one. It looks like it is dying from thirst even though It's getting drip irrigation water every day. Could the cause be the scenario you describe? There are still 4 other jasmins planted the same way (in little openings in pavers) that seem to be doing well.