I have had a tombstone dissippear. I was fighting on a slope right where a mountain and plains biome met, in the middle of the day, and got overwhelmed. It was a distinctive spot and I luckily had a portal nearby, so went back right away with my backup armor and weapons. Still lots of daylight. Finished off the few enemies still around and then looked for my tombstone. No tombstone to be seen anywhere. Double-checked map. Skull marker on the map right where I was standing. Walked around a bit and still didn't see it. Finally, just because I couldn't think of anything else to try, went to the exact spot I thought I had fell at, pulled out my pickaxe, and started digging. Bingo! Found it. It was just under the ground, like the top was barely below the sloping surface.

So go back to the spot in the daylight and try using a pickaxe to dig where the death marker is on the map. That may lead you to it.

If that doesn't succeed, then I would suggest contacting the Developers. They may have a way you can recover it or a previous save.

Cheers.