A photo would help but it's quite probably just the natural process of the flowers dying of old age - could be made worse by too much sun, too much wind, too much water, not enough water....

I had the same problem with my Hydrangea. The flowers on the one in my back garden, which is south facing, started to look wilted and dried, but the one in the front (north facing) seemed ok.

I assumed that this was due to the unusual hot weather we've been having and each night I have been spraying the flowers and leaves with a hosepipe. After two days it perked up and I now have a lovely show of lovely pink blooms.