It's possible that your hedge has been attacked by the Ermine Moth. In April/May you would have seen webs in the hedge, a bit like a spider's web in frost, from within which caterpillars eat the hawthorn leaves to the point where the hedge appears to have died. If this was the case, the hedge will recover nextyear. I had such an occurrence foir years ago aand when I spotted the webs the following year I sprayed with Dipterex 80 and had no further problems although I still keep a sharp lookout at that time of year. It seems that hedges which are well cared for and clipped are more prone to these attacks.