You can soon find out if it is dead by snapping off a small twig, as although it may look dead, if the twig is still green in the middle the plant is alive and may well shoot into life once the warmer weather kicks in. Assuming the bush is alive, lightly fork in some good quality well rotted manure and/or fertiliser around the base of hedge so that it has a good feed to get it going once it begins to shoot. Any foliage that appears could well benefit from a spray of liquid seaweed, or you could give the hedge a good watering with the liquid seaweed solution.
I am not familiar with 'Gresilena' specifically, but have you considered pruning it too, as this often encourages new growth and production of new and younger 'wood' on many plants and shrubs.
Above all be patient, many people assume a tree or shrub has died, only to see it miraculously return to life when it gets a few days of sunshine.