This is probably frost damage, assuming that the plant is in the garden and not in a pot. New leaves will grow later.

Just wondering how long you have had it, if it should be a new one bought this spring, I wouldn't be surprised this is happening. The same thing happend to mine which I bought from Homebase. They told me, that they were forced on ready for Mothers day in March, therefore they then reduced them but they would be fine for next year. This from the Manager - I was so angry, o.k. they were a couple of pounds cheaper, but hoped they would last just a bit longer at least. If you had yours longer, ignore the above, but I think too exposed to the sun is more likely, I have quite a few Hydrengeas and they grow much better in a place where they are in part sun part shade.

Many thanks. No, had them ab. 3 yrs and bought from a reputable nursery.
The general consus seems to be frost or certainly a severe drop in temp.