Please act quick, this looks like typical damage caused by the conifer aphid, which feeds on the woody growth aprox three inches from the tips of the foliage, you shuold see signs of sooty mould, and if you rub your hands in the foliage you should see signs of the aphid, assuming it is this then buy one of them sprayers and spray copiously on all sides use buglear concentrate, from most diy stores and spray three/four times a year, please note once conifer dieback occurs then it will not come back, if not conifer aphid then look for conifer scale, julien.

Oh what a shame as you obviously look after it. The RHS have a page of information on browning conifers/hedges.

Many thanks