As a horse lover and owner for many years this is shocking! I had this happen to me when I loaned a pony to a man my family knew well for his grand daughter and they up and left never to be seen again. I kept looking for 5 years, finally found someone who had purcahased the horse at a sale yard and got a new trail, 2 more years and found my pony at ridding for the disabled. When I produced the paperwork and police reports they offered to return him but I opted to leave him there as he was to small for me to ride anymore and the kids loved him.
Never give up hope and keep going, try everything and when it does not work try again. I really so hope that you get your horse back