Couple of questions - have you checked the base of the plant to check if the soil is a bit dry? 'this assumes it is in the ground and not a pot.
And are you saying the whole thing is drooping, or just the growth on the top and outside, but particularly the top?

Mine drooped in the drought and recovered when I watered it.

I have found them to be very subject to root rot in poor drainage. They might go on for a few years, but then your luck runs out, and the necessary fungus becomes established. Usually once one has wilted, there is no point in planting more within 10 meters.

Thanks for that Tub, do you know why my flower heads have suddenly turned to mush on one of mine?

Sudden impact of frost, I expect, Drc - which is what I was wondering about Valerie's - the flowers are usually sticking out at the top and sides, and thus are more vulnerable to frost, and we had a pretty hard frost here in London 2 nights ago, so yours was probably worse. I notice also that its not just the flowers, its often the leaves immediately below the flowers and the stems which carry them.
However, unless Valerie answers, we'll never know...

Thanks Bamboo I am sure you are right about frost we have had a couple.