Sudden unexplained fearful behavior

Question
Hello  -  I am in need of expert advice, please, about a unique situation.
I have had my 15 year old Thoroughbred gelding for nearly 3 years.  He is my only horse and the love of my life.  However, for the past 3 days, he has suddenly been acting fearful of me.  He is normally a VERY docile, trusting, "bombproof" companion who is comfortable being touched/handled all around on a daily basis.  Yet now, for some unknown reason he backs away to keep his distance and snorts as though he's afraid to be approached/touched either by me or even by others who he has known/trusted much longer than me.  There doesn't appear to be any reason for this!  He is in great health and current on all exams/innoculations.  There have been no changes to his diet, appetite, surroundings or routine.  He is stabled in a great environment with 2 acre turnout and full run of the facility.  He is the highlight of my day - EVERY day, and so this sudden, unexplained fearful behavior troubles me deeply - to say nothing of the way it has obviously traumatized him.  I am completely baffled as to what caused this or how to deal with it. For lack of better explanation, I'm inclined to suspect that someone has somehow done something mean to him when I'm not around. On that note, I am taking steps to install security cameras.  Meantime, I am determined to work with my dear companion and do whatever is necessary to regain his trust.  Any techniques or advise you could offer on that front would be MOST sincerely appreciated.  

Thank you in advance, and have a wonderul day!

Answer
Two things come to mind, an eye or sight issue when horses go blind or have eye issues they get scared easy and tend to appear spooky.

The other is someone has hit or beat the horse, I would have everyone who handles or access to horse to give him a treat and watch the horse reactions, the horse will tell you.