A photo would be useful but from what you say, its likely drought. If you put it in a pot with drainage holes, then overwatering is not really possible because excess drains away through the holes. This particular variety of hydrangea doesn't really need ericaceous compost, though it won't do any harm. The flowers turn pinkish then reddish as they fade - if yours were turning from white prior to potting up, they might have been on the plant longer than you realised, or it might be caused by drought, because without sufficient water, flowers will not bloom for so long as they would with sufficient water.
Water when the surface of the soil feels just about dry to the touch, but not so dry its shrunken away from the sides of the pot and water thoroughly. I don't know what size pot its in, so hard to say how many litres to give it, but if its dry now, soak it thoroughly till the soil is wet all the way through. And water the soil, not the foliage... Bear in mind that rain often doesn't get into the soil of a pot - its kept out by foliage, so you will still need to check the soil and water as necessary, even on a day when there's showers about..
I've assumed its in a pot- if you planted it in ericaceous compost you put in a hole in the ground, leave the hose trickling at the base for an hour or two, moving it two or three times to cover the soil round the base of the plant and keep it well watered up till late autumn.

Thank you. Its in the ground (transferred from the pot when bought). It has been watered daily so I'll just have to keep my eye on it. The thing is, I have quite a lot of Hydrangeas and always have had as I like them very much (now have large garden) and I have never had any problem which is why I really didnt know what is wrong. Thanks again.

Its much, much harder to keep a new plant properly watered in the ground at this time of year than it is in a pot, especially a hydrangea which does not like to dry out .. it might have been better to keep it in a bigger pot until autumn, then plant.

Yes Bamboo, perhaps I was a bit eager to get it in the ground. However, its looking on its way to getting better now I've nearly drowned it. Just one stem seems to be irreparable but at least the plant is saved, Thank you again.

You could consider carefully digging it back up and potting it till late September - it won't have put out any roots yet, will still be pot shaped at the bottom!

Ah worth a try. Its strange actually, because I bought 4 different hydrangeas at the same nursery and the same day. All planted the same day, in the same area of my garden, all white in colour. Two of them were simply bog standard hydrangeas without even any name or instruction card and are doing brilliantly. One other is another white paniculata 'Silver Dollar' (I am doing a white and green area in the garden). All three of those are doing great. Really strange. Thankyou for being so helpful.

That is a bit odd, if 3 are doing fine and Candlelight isn't - could be a dodgy plant,or maybe its planted in a drier spot or a spot that gets a bit more sun than the others...

No same area, same soil, same sun, same watering. I'm beginning to think you may be right Bamboo.

Well, it is often surprising - we think the area is exactly the same as another plant occupies,but it won't be, exactly, all sorts of things may be slightly different and we're unaware of them. The fact it recovered somewhat after a thorough watering would suggest it was a drought problem...