Ask-a-Dave's-Gardener: What has happened to my hedge? Can it be saved?, 1 by LBB2121

LBB2121 wrote:
I have 5 well established hedge trees (James Stirling?) along my driveway (approx 15 years old) Over the past 6 months the middle tree has begun to die (is possibly dead?) and the 2 on the left are also heading that way :(
The 2 on the right are still lush and green.
What may have happened? Can they be saved?
The only watering they recieve is rain but this has been sufficient for many years.
Please help!