Trolls can't destroy gravestones, but they might knock it away a bit especially if you died on a steep hill. Happens in the mountains sometimes. Look around down hill, maybe it's there.

Others have been losing them inside dungeons though. Were you inside a troll cave? If it fell through the floor you probably aren't going to get it back.

But you're early in the game. Go after the troll with a bow and slow it down by running between trees and jumping over logs, or cheese it by digging a trench and shooting across it. They move fast in open areas, but get slowed down by having to go around obstacles that you can jump over or fit through in the forest, just watch out for falling trees. Troll armor is better than the stuff you lost.